Three pipes, A, B, C can fill an empty cistern in 2, 3 and 6 hours respectively. They are opened together. After what time should B be closed, so that the cistern gets filled in exactly 1 hr 15 min?

30 min

A+ = 2 hrs, B+ = 3 hrs, C+ = 6 hrs,

⇒ Let B be opened for t hours and A + C for \(\frac{5}{4}\) hrs,

According to the question,

⇒ (3 + 1) x \(\frac{5}{4}\) + 2t = 6,

⇒ 5 + 2t = 6,

⇒ 2t = 6 - 5 = 1,

⇒ T = \(\frac{1}{2}\) hrs.

Therefore, B should be closed after 30 min.