A poplar tree was 3 feet high when it was planted on January 1, 1970. During what year did it pass the height of 20 feet

[M7MBA]

A poplar tree was 3 feet high when it was planted on January 1, 1970. During what year did it pass the height of 20 feet?

(1) On January 1, 1973, it was 24 feet high.
(2) It doubled its height each year.

Answer: B

Source: GMAT Prep

[Brent@GMATPrepNow]

A poplar tree was 3 feet high when it was planted on January 1, 1970. During what year did it pass the height of 20 feet?

(1) On January 1, 1973, it was 24 feet high.
(2) It doubled its height each year.

Answer: B

Source: GMAT Prep

Given: A poplar tree was 3 feet high when it was planted on January 1, 1970.

Target question: During what year did it pass the height of 20 feet?

Statement 1: On January 1, 1973, it was 24 feet high.
Important: This means the tree grew 24 feet in 3 years. HOWEVER, we cannot then conclude that the tree grew 8 feet each year, since we're given no information to suggest the tree grew at a constant rate.
This means the tree's height could have passed 20 feet in 1970, 1971, or 1972
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: It doubled its height during each year.
Perfect! We now know the following:
January 1, 1970: height = 3 feet
January 1, 1971: height = 6 feet
January 1, 1972: height = 12 feet
January 1, 1973: height = 24 feet
So, the tree's height passed 20 feet in 1972
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer: B

Cheers,
Brent