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Virtually every ordinary fraction simply represents division of the numerator by the denominator, so to convert a common fraction to a decimal, just divide the numerator by the denominator in a column or on a calculator. For example, try to translate 2/3. Divide 2 by 3, you get a recurring decimal 0.6666(6).
The second method is to translate a common fraction into a decimal is to multiply the numerator and the denominator by the whole number to the denominator turned 10 in any degree (10, 100, 1000, etc.). It is not always possible, so for the fraction 2/3 such integer does not exist, but for a fraction 7/125, there is a multiplier. Is 8.

125 * 8 = 1000, then 7/125 = 7*8/1000 = 56/1000 = 0.056.