Let
A = population of town A,
B = population of town B,
C = population of town C.
Because the population of town B is 50% greater than that of town A,
B = 1.5A (1)
Because the population of town C is 20% greater than that of town A,
C = 1.2A (2)
From (1), obtain
A = B/1.5 = (2/3)B (3)
Substitute (3) into (2).
C = 1.2*(2/3)*B = 0.8B
Divide each side by 0.8.
1.25C = B
This result means that B is 25% greater than C.
Answer: 25%
