25 percent of a class are girls - there are 13 girls - how many are in the class?

25% = 1/4, so 13 x 4 = 52

13 ÷ 25%

= 13 ÷ 25/100

= 13 x 100/25

= 1300/25

= 52 people

The above is a good answer (and correct in the solution) however, I feel the explanation of how the problem is solved isn't the most accurate way to solve this problem. As you stated 25% is 1/4, however, using this method to solve this problem will prove more complicated if the percentage becomes a more complicated number such as 33% or 35%, just a couple of examples.

(always remember to change your percentage to decimal form before solving)

The best way to solve such an equation is to remember the rules of Part, Base, Rate.

Rate always being the percent (%)

Remember to find the part the solution is b * r(%) = p

Example, 25% of 100 would be 100 x .25 = 25

If you have a problem where the part and the rate are given, then you are looking for the base, or your starting number. As with the problem above, we know that 13 is the percentage of how many students are girls in a class, however we now need to solve to find out exactly how many students are in the class, since we are given the p (part) and the r (rate) we need to solve for base.

b * .25 = 13.

reverse the order (b = p / r)

b = 13 / .25

b = 52

To check replace b with 52 and solve

52 * .25 = 13

Now let's take the same problem and search for rate if base and part are given.

since we know the problem is b * r(%) = p, the we write it as follows

52 * r = 13

reverse the order once again, as with the first problem, remember this time we are searching for the rate. The equation would then be written (r = p / b)

r = 13 / 52

0.25 = 13 / 52

Change the decimal .25 to a percentage by moving the decimal two places to the right...

25%

to check just replace the original problem with your answer and solve...

52 * .25 = 13

Remember the 3 basic rules,

b * r = p

p / b = r

p / r = b