a. Using the ratio of the sides 12 cm and 16 cm
x/20 = 12/16
x = (12(20)) / 16 = 15
b. Using the ratio of the sides 6 cm and 8 cm
x/20 = 6/8
x = (6(20)) / 8 = 15
c. The answer is the same since the ratio 12/16 and 6/8 is both equal to ¾.
Hope this answer will be a good help for you.