Question

The ratio of blue marbles to yellow marbles in a jar is 7 to 9. If there are 28 blue marbles in the jar, how many yellow marbles are there?

Answer 1

There are 36 yellow marbles.

proportion: a:b = c:d where ad = bc

7:9 = 28:x

7x = 9*28
7x = 252
7x/7 = 252/7
x = 36

7:9 = 28:36
7*36 = 9*28
252 = 252

Answer 2

Ratio:  7:9

Blue marbles to yellow marbles. 

28 blue marbles 

7*4 = 28
9*4 = 36

28:36

Answer: There are 36 yellow marbles