At the bank of a river, by measurement, a = 9 ft., b = 15 ft., c = 12 ft., and d = 7 ft. How long is x?

Answer: x is 20

Step-by-step explanation:

This is ratio

a : c = b : x and d : unknown

a : c = 9 : 12 = 3 : 4 which is 9/12

b : x = 15 : x which is 15 / x

So the proportion will be:

3/4 = 15/x

So Cross multiplying

3x = 90

Divide both side by 3

x = 90/3

x = 20