If population of certain city increases at the rate of 5%. If population in 1981 was 138915, then population in 1978 was__________?

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If population of certain city increases at the rate of 5%. If population in 1981 was 138915, then population in 1978 was__________?

A. 1,20,000
B. 1,10,000
C. 1,00,000
D. 90,000
Explanation:
X * (105/100) * (105/100) * (105/100) = 138915
X = 138915/1.157625
X = 120000