Method 1 (works for any %)
you can use the algorithm is/of = %/100
so .. 21/x=25/100
cross multiply you get 2100=25x
now divide both sides by 25 you get 84
Method 2 (works for percents that are easy fraction like 25%, 10% etc)
you can also see that 25% is 1/4... so if 1/4 is 21 then what is 4/4?
4 times 21 is 84
