Answer :

iGreen
Divide the numerator and denominator by 21:

21 / 21 = 1
84 / 21 = 4

So 21/84 is equal to 1/4.
21/84

21 divided by 3 = 7
84 divided by 3 = 28

28 divided by 7 = 1/4 

1/4 is your answer