Turn one pipe off after an initial burst Two pipes A and B can fill a cistern in 15 minutes and 20 minutes respectively. Both are opened together; after exactly 2 minutes, A is closed. What total time is required to fill the tank completely?

Introduction / Context:
Phased-operation problems require computing volume added in each phase and then finishing the remainder with the active pipe’s rate.

Given Data / Assumptions:

• A: 15 min ⇒ 1/15 per min.
• B: 20 min ⇒ 1/20 per min.
• Phase 1: both run for 2 min; Phase 2: only B runs to completion.

Concept / Approach:
Work after Phase 1 plus work in Phase 2 must equal 1 tank. Solve remaining fraction / B’s rate.

Step-by-Step Solution:

Verification / Alternative check:
Back-substitute: (2 min at 7/60 per min) + (46/3 at 1/20) = 1 tank.

Why Other Options Are Wrong:
52/3 is the sum if you misinterpret the split; 43/3 and 41/3 miscompute the remaining fraction.

Common Pitfalls:
Adding 2 minutes twice or using average times.

Final Answer:
46/3 min