Step 1: Define rates
- Tap A fills in 60 min → rate of A = \(\frac{1}{60}\) tank/min
- Tap B fills in \(x\) min → rate of B = \(\frac{1}{x}\) tank/min
Step 2: Combined rate
Together they fill in 40 min → combined rate = \(\frac{1}{40}\) tank/min
Step 3: Set up equation
\[
\frac{1}{60} + \frac{1}{x} = \frac{1}{40}
\]
Step 4: Solve for \(x\)
\[
\frac{1}{x} = \frac{1}{40} - \frac{1}{60} = \frac{3 - 2}{120} = \frac{1}{120}
\]
\[
x = 120
\]
Thus, Option (D) 120 min is correct.