A swimming pool has two inlet pipes. The first inlet alone can fill the empty pool in 21 hours, and the second alone in 28 hours. The pool is empty and both inlets are run together. In how many hours will the pool be full?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 12 hours
Take the capacity of the pool as the LCM of 21 and 28, that is 84 units. Pipes running together add the units they fill each hour.
First inlet: \(84\div21=4\) units per hour
Second inlet: \(84\div28=3\) units per hour
Both together: \(4+3=7\) units per hour
Time to fill the pool \(=84\div7=12\) hours
The pool is full in 12 hours.