21 is 75% of what amount

In the given question it is required to find an unknown amount from all the details given in the question. It is given that 21 is the 75% of the unknown number.
Let us consider the amount = x
Then we can write the equation as
75x/100 = 21
75x = 21 * 100
75x = 2100
x = 2100/75
  = 28
So the correct amount is 28. . I hope to have made the procedure clear for finding the solutions of such problems.

21 is 75% of 28. You already know that to make 100, you need 4 25's. 75 is three 25's. You take 21 and divide it by three to get seven. Then you take your past knowledge and multiply 7 by 4 and get 28.