How do you get 25% of what number is 21

You just needed to do is 25% and then times it by 21 you should get 5.25

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

40%=2/550=1/230=3/1056=14/25