A tank can be filled by Tap 1 in 20 minutes and by Tap 2 in 25 minutes. Both taps are opened together and run for exactly 5 minutes, after which Tap 2 is turned off. From that moment, how many additional minutes will Tap 1 alone take to completely fill the tank?

Introduction / Context:
This is a two-phase filling problem. We compute the fraction filled in the first phase (both taps on), then determine how long the remaining fraction takes with only Tap 1 working.

Given Data / Assumptions:

• Tap 1 rate = 1/20 tank/min.
• Tap 2 rate = 1/25 tank/min.
• Both on for 5 minutes, then Tap 2 is closed.

Concept / Approach:
Phase 1: fraction filled = time * (rate1 + rate2). Phase 2: remaining fraction divided by rate1 gives the additional minutes required.

Step-by-Step Solution:

Verification / Alternative check:
In 11 min, Tap 1 adds 11/20; total becomes 9/20 + 11/20 = 1, as required.

Why Other Options Are Wrong:
17 1/2, 12, 6 min do not match the exact remainder and Tap 1 rate.

Common Pitfalls:
Computing the 5-minute fill incorrectly or forgetting to subtract to get the remainder.

Final Answer:
11 min