The number of chickens on Alice’s farm triples every year.

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[GMAT math practice question]

The number of chickens on Alice's farm triples every year. At the beginning of the first year, Alice bought a certain number of chickens. She sold 25 chickens at the end of years 1 and 2. After 3 years she had 240 chickens. How many chickens did Alice originally buy?

A. 15
B. 20
C. 25
D. 30
E. 35
Source: — Problem Solving |

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by Max@Math Revolution » Thu Aug 08, 2019 11:37 pm
=>
Suppose n is the number of chickens that Alice started out with.
After one year, Alice had 3n - 25 chickens.
After two years, Alice had 3(3n-25) - 25 = 9n - 100 chickens.
After three years, Alice had 3(9n-100) = 240 chickens.
So, 9n - 100 = 80, and 9n = 180.
Thus, n = 20.
Alice originally bought 20 chickens.

Therefore, B is the answer.
Answer: B