Time for partial fill with three inlets Taps A, B and C fill a tank in 20 min, 15 min and 12 min respectively. If all are opened together, how long will they take to fill 40% of the tank?

Introduction / Context:
Partial-fill questions use the same rate addition as full-fill problems, but the target volume is a fraction (here 40%) of the tank.

Given Data / Assumptions:

• A: 20 ⇒ 1/20 per min; B: 15 ⇒ 1/15 per min; C: 12 ⇒ 1/12 per min.
• All open from empty; target volume = 0.40 tank.

Concept / Approach:
Combined rate = sum of individual rates. Time = (target fraction) / (combined rate).

Step-by-Step Solution:

Verification / Alternative check:
In 2 min, they add 2 * (1/5) = 2/5 = 40%.

Why Other Options Are Wrong:
1, 3, 4 min correspond to 20%, 60%, 80% respectively at this rate.

Common Pitfalls:
Computing each tap’s 40% time separately and averaging—incorrect.

Final Answer:
2 min