Question

Two pipes A and B can fill a cistern in 30 min and 40 min respectively. Both the pipes are turned on simultaneously. When should the second pipe be closed if the cistern is to be filled in 24 min. 

(a) 6 min 

(b) 8 min 

(c) 10 min 

(d) 12 min

Answer 1

(a) 6 min

Let the second pipe be closed after x minutes.

Then, \(\frac{24}{30}+\frac{X}{40}\) = 1

\(\Rightarrow\) \(\frac{96+3X}{120}\) = 1

\(\Rightarrow\) 3x = 120 – 96 = 24

\(\Rightarrow\) x = 6 min