A bathtub can be filled by the cold water faucet in 15 minutes and by the hot water faucet in 18 minutes. How long does it take to fill the tub if both faucets are open?

Question

Quizplus · Accepted Answer

Let the volume of the bathtub be 1 unit (we can assume any unit) 
The cold water faucet fills the bathtub in 15 minutes, that means it fills 1/15 of the bathtub in 1 minute. 
The hot water faucet fills the bathtub in 18 minutes, that means it fills 1/18 of the bathtub in 1 minute. 
When both faucets are open, the volume of water filled in one minute is (1/15 + 1/18) units. 
Simplifying, (1/15 + 1/18) = 0.2 + 0.16 = 0.36. 
Therefore, the time taken to fill the bathtub when both faucets are open is (1/0.36) minutes 
= 2.7777... minutes, which is approximately equal to 2.78 minutes. 
Therefore, the best choice is B.

A Bathtub Can Be Filled by the Cold Water Faucet