Gmat_mission wrote: ↑

Thu Aug 20, 2020 9:33 am

A box contains only red chips, green chips, and blue chips. If a chip is randomly selected from the box, what is the probability that the chip is either red or green?

A. The probability of selecting green chip is 1/3

B. The Probability of selecting a blue chip is 1/7

Answer: B

Source: Magoosh

Target question: What is the probability that the chip is either red or green?

Given: The box contains only red chips, green chips and blue chips

Statement 1: The probability of selecting green chip is 1/3
This tells us that 1/3 of the chips are GREEN, but there's no information about the RED chips.

Consider these two possible cases:
Case a: There are 6 chips in total. 2 chips are green, and 2 chips are red. In this case, the answer to the target question is P(chip is red or green) = 4/6
Case b: There are 6 chips in total. 2 chips are green, and 3 chips are red. In this case, the answer to the target question is P(chip is red or green) = 5/6
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: The probability of selecting a blue chip is 1/7
This also tells us that the probability of selecting a NON-BLUE chip is 6/7
Since the box contains only red chips, green chips and blue chips, then selecting a NON-BLUE chip is the same as selecting either a RED chip OR a green chip
In other other words, P(select NON-BLUE chip) = P(selecting either a RED chip OR a green chip)
So, the answer to the target question is P(chip is red or green) = 6/7

Answer: B

Cheers,
Brent