Tell your child a story about how the word percent. It comes from the Latin "pro centum", which means "hundredth". Later in the tutorial, Mathieu de La Porte, commercial arithmetic were made a typo, where and a % sign. Thus, the most important thing is to understand that the cent is one hundredth part from any number.
Usually, the child quickly understands the task on the primes. For example, one ruble is 100 kopeks, 50 cents is 50 percent. Much harder to explain that the interest can be of any size. Having dealt with simple variables: grams and kilograms, centimeters and meters – move on to more complicated issues.
If the child can not understand the essence of interest, teach him to solve problems according to the algorithm, making sure he didn't miss a one step solution. For example, the task: sewing factory was released in the year 1200 suits. 30% of them suits blue. How many suits of blue color has been released by the factory? First, find how many suits make up 1%. To do this, divide the total number by 100. 1200/100 = 12. That is, every 12 costumes is 1 percent. Then multiply 12 by 30% and get the desired response.
You can use the old "grandfather's" method of proportion. In schools now for some reason it rarely shows, and it works flawlessly. From the same task:
1,200 costume – 100%

X costumes – 30%

X (1200 * 30)/100.
You just need to multiply the numbers crosswise and solve the resulting equation. Don't worry if you think that the child solves mechanically. While he doesn't have to deeply ponder the essence, the most important thing to remember sequence of actions, that will be enough to solve school problems. Be patient, do not yell at the child and not be angry at him. Because he feels that this information is very complex, confusing and not right. Try to offer him practical tasks, for example, for the family budget.