The product of 25 percent of 100 is 25. The best way to calculate percentages is to use what mathematicians call a proportion, which states that two ratios are equal to each other. All percentages can be converted into ratios using fractions in the following way: x % = x/100.
When using proportions to calculate percentages, one must first set up the proportion. In this case, x/100 = 25/100. It may be easy to see that x = 25, but the correct method is to multiply across the equal sign to remove the fractions. Then the result is 100x = 2,500, which simplifies to x = 25.