A tyre has 3 punctures. The first puncture alone can let out all the air in 7 minutes, the second alone in 14 minutes and the third alone in 28 minutes. If the air lets out at a constant rate, how much time (in minutes) will it take for all the punctures to let out all the air from the tyre?
Show answer & explanation
Correct answer: C
Explanation
The correct answer is option 2 i.e. 4.
Let the total air to be let out (LCM of 7,14 and 28) be 28 units.
Efficiency of puncture 1 = 28/7 = 4 units/minute
Efficiency of puncture 2 = 28/14 = 2 units/minute
Efficiency of puncture 3 = 28/28 = 1 units/minute
Time taken by all 3 punctures together to let the air out = 28/(4 + 2 + 1)
⇒ 28/7 = 4 minutes