21/12 = 7/y?

21/12 = 7/y

simplify first

7/4 = 7/y

divide through by 7

1/4 = 1/y

take the reciplical of both sides

4 = y

21/12 = 7/y
(21/3)/(12/3) = 7/y
7/4 = 7/y
7y = 28
y = 4

Answer: y = 4

Proof:
21/12 = 7/4
7/4 = 7/4

y=4

on top, 21 is 3 times 7, so on bottom, 12 is 3 times y, which makes y = 4

you can always cross multiply to solve the proportion:
21y = 12(7)
y = 12(7)/21 = 12/3 = 4

21/12 = 7/y
21y = 84
y = 4

cross multiply first..
21/2=7/y
21y=7 x 12
21y=84
y=84/21
y=4

21 y = 84
y = 4

21/12 = 7/y
21y/12 = 7
21y = 84
y = 84/21
y = 4

y = 4
so
21/12 = 7/4

21 divided by 7 = 3
so 12 divided by 3 = y
y = 4