Water flows through a pipe of radius 7 cm at a speed of 2 m/s into an empty cylindrical tank of…

Step 1. The pipe. The radius is $7\ \text{cm} = 0.07\ \text{m}$. The cross-section is $\dfrac{22}{7} \times 0.07^2\ \text{m}^2 = 0.0154\ \text{m}^2$.

Step 2. Water each second. $0.0154\ \text{m}^2 \times 2\ \text{m/s} = 0.0308\ \text{m}^3/\text{s}$.

Step 3. The tank. $\dfrac{22}{7} \times 1.4^2\ \text{m}^2 \times 2\ \text{m} = 12.32\ \text{m}^3$.

Step 4. The time. $12.32 \div 0.0308 = 400\ \text{s}$, which is $6\frac{2}{3}$ minutes.