# Advice 1: How to calculate interest on a calculator

To calculate the percentage ofs we have often. For example, to determine the overpayment on the loan or calculate interest on arrears, or to know the gross margins of the company, knowing its turnover and trading margin. Such a lot of tasks in daily life. It would be funny every time to make the proportions and percentages in a column. Moreover, for calculations we do not need a complex programmable calculator. Instruction
1
Clearly specify the problem statement. We need to find 7% of the number 594.
2
The digits on the calculator number, which is necessary to calculate the interest. In our case, the dialed 594.
3
Click the "X" button. Is - the button for multiplication. Do not confuse it with the " + " button (add).
4
Enter the percent. In our case, type the number 7.
5
Click the "%". This is the letter of interest. More than anything don't type it, no characters "=" (equals). The calculator immediately shows the calculated value. In this case, the figure turned 41,79. Thus, 7% of the number 594 = 41,79.
6
Click "S". This is the reset button, it is highlighted in a different color. The calculator will display zero and you can do the following calculation.
Note
Do not use calculator which works intermittently. Calculator - working tool, the correctness of the testimony which much may depend. Purchase once a good device that will last you for years to come. Reliable calculators are produced by well-known companies producing electronic equipment.
Useful advice
Sometimes check calculate a normal column, not to forget how to count. After all, at hand may not have a calculator. You don't have to get into an awkward situation. Remember that simple calculators built into cell phones.

# Advice 2 : How to subtract percentages

A percentage of a number is a hundredth of this number, denoted by 1%. One hundred percent (100%) equal to the number, and 10% of the number equal to a tenth of that number. Under the subtraction percent decrease understand the numbers on what the share. You will need
• Calculator, paper, pen, skills of oral accounts.
Instruction
1
Turn on the calculator and enter a number N from which you want to deduct a percentage.
2
Press the " - " sign, then dial M percentage you want to subtract and press "%", then press "=". As a result, you will receive a number that is less than the number N by M percent.
3
If you don't have a calculator, divide the number N by 100. So you will receive the part number, which is 1 per cent. Next, the resulting, after dividing, multiply by the number M. as a result, you will receive a part of the number K, which accounts for M%. Then subtract from the initial number N number K, which is equal to M percent of the number N. the result of subtraction, you will receive a number that is less than N by M percent. That is, you deduct from the number a fraction of a percent.
Note
The percentage deducted may not exceed the value of 100. The interest rates vary from zero to one hundred.
Useful advice
The same way you can to a number and to add interest. To do this, instead of the key with the sign "-", press the "+"sign.

# Advice 3 : How to calculate percentage in mathematics

With the need to calculate the percent of people faces constantly, sometimes even without realizing it. And not only on the math, but, for example, trying to determine what part of total family income to make utility payments or payment for a kindergarten. And many need to count the percent baffled. Instruction
1
Understand that the number from which to calculate percentages is always one hundred percent. Regardless of whether it is in the problem or you found it, adding the income of all family members. It can be described, for example, the letter a or any other letter, and you do not designate.
2
Find that number 1 percent. For this original number, divide by 100. If we take the General formula, 1 percent of the number and will be equal to a/100.
3
Suppose you want to find not 1 percent and 20. Then the number which designated 1 percent of the specified, must be multiplied by the required percentage. That is, will a/100*20. For example, your salary is 11 000 rubles. 1% of this number is 110 rubles, and 20% -2 200 rubles.
4
For example, the problem is to know how many percent of the original number a is number b. The amount of interest that you want to find, label as X. Find 1%, which will be a/100. In order to determine how many percent of the number a is number b, it is necessary to divide it into the value. Remember the rule of dividing the whole number into a fraction. Need the number to divide into the numerator of the fraction (in this case a) and multiply by its denominator (in this case, 100): x=b/a*100. For example, at the warehouse brought 250 bags of potatoes. 35 storekeeper immediately sent to the store, and he needed to know how much it amounts to per cent of the total number of bags. Find 1% which in this problem will amount to 250/100=2.5 bag. Dividing 35 by 2.5, you get the desired amount to 14%.
Note
Before you take on the task of interest, remember the rules of multiplying and dividing fractions.
Useful advice
When calculating interest, you can use the calculator. But you can experience infinite decimal fraction. It must be rounded.

# Advice 4 : How to solve problems on proportions

Leaves no doubt that the proportions of the thing desired. The proportions in our lives everywhere. Calculate salary for the year, knowing the monthly income. How much to buy of goods for money, if you know the price. It's all proportions. Instruction
1
When solving problems on proportions is always possible to use the same principle. That they are comfortable. When dealing with proportion, always proceed as follows:Define the unknown and label it with the letter H.
2
Record the condition of tasks as a table.
3
Determine the type of dependence. They can be direct or reverse. How to determine the type? If the proportion is subject to the rule "the more, the better", so a direct relationship. If on the contrary, "the more, the less", it means an inverse relationship.
4
Put your hands with the edges of the table in accordance with the type of dependency. Remember: the arrow pointing upwards.
5
Using the table, write a proportion.
6
Solve the proportion.
7
Now let us examine two examples of different types of dependence.Task 1. 8 yards of cloth cost 30 R. How much are the 16 yards of this cloth?
1) the Unknown - the cost of 16 yards of cloth. Let's denote it as x.
2) let's Make the table:8 yards 30 p.
16 yards x R. 3) to Define the type of dependency. Think: the more cloth you buy, the more you will pay. Therefore, a direct relationship.4) Put the arrow in the table:^ 8 yards 30 p. ^
| 16 yards R. x |5) we form the ratio:8/16=30/xx=60 p answer: the cost of 16 yards of cloth is 60 p.
8
Task 2. The motorist noticed that at a speed of 60 km/h he drove a bridge across the river for 40 s. On the way back he passed the bridge in 30 s. Determine the speed of the car on the way back.1) the Unknown - the speed of the car on the way back.2) let's Make the table:60km/h 40
x km/h 30 C3) Define the type of dependency. The greater the speed, the faster a motorist will pass a bridge. Hence the inverse relationship.4) will form the proportion. In the case of inverse relationship here a little trick: one of the table columns you need to flip. In our case, we get the following proportion:60/x=30/40x=80 km/cotvet: back on the bridge a motorist drove at speeds of 80 km/h.
Useful advice
A direct correlation obeys the rule of "the more, the better".

Reverse obeys the rule of "bigger is less."

In the case of reverse dependencies when preparing the proportions of one of the columns of the table should turn.

# Advice 5 : How to calculate mass percent

Mass percent is the ratio of the mass of any component of the solution, alloy or mixture to the total weight of the substances in this solution, expressed in percentageAh. The higher the percentage, the more the content of a component. Instruction
1
Remember the task set before the great scientist Archimedes king Hieronim, and slightly modify it. Suppose Archimedes found that the roguish goldsmith had stolen some of the gold, replacing it with silver. As a result, the alloy from which were made the Royal crown, consisted of 150 cubic centimeters of gold and 100 cubic centimeters of silver. Task: find the mass percentage of gold in the alloy.
2
Remember the density of these precious metals. 1 cubic cm of gold contains 19.6 grams of 1 cubic cm of silver is 10.5 grams. To simplify, you can round these values to 20 and 10 grams, respectively.
3
Next, perform the calculation: 150*20 + 100*10 = 4000 gram, that is 4 pounds. This is the alloy used for fabrication of the crown. Since the problem says nothing about the "waste" will get the response: 150*20/4000 = 3/4 = 0,75. Or in another way, 75%. That was the mass percentage of gold in a supposedly "pure gold" the crown of Hiero.
4
And if you were dealing with a solution? For example, you have been given this task: to determine the mass percentage of salt (sodium chloride) in it dvuhosnom solution.
5
And there is absolutely nothing complicated. Remember, what is the molarity. This is the number of moles of substance in 1 liter of solution. Mol, respectively, the amount of a substance whose mass (in grams) equal to its mass in atomic units. You have only to write the formula of common salt, and learn a lot of its components (in atomic units), looking at the periodic Table. The mass of sodium is 23.e.m. the mass of chlorine is 35.5.e.m. The amount you get of 58.5 grams/mol. Accordingly, the mass of 2 moles of salt = 117 grams.
6
Therefore, in 1 liter of 2M aqueous solution of sodium chloride contains 117 grams of the salt. What is the density of this solution? According to the table of densities, find that it is approximately equal to 1.08 g/ml. Therefore, in 1 liter of this solution will contain about 1080 grams.
7
And then the problem is solved in one step. By dividing the mass of salt (117 grams) the total weight of the solution (1080 g), we get: 117/1080 = 0,108. Or in the percentage ofOh is 10.8%. This is the mass percentage of salt in its 2M solution.
Note
Mass percentage of sodium chloride will remain unchanged, regardless of the number of the solution.
Useful advice
When solving the problem with the gold alloy you used the rounded value of the density of gold and silver. If you want more precision, this is unacceptable.
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