A train travelling at a speed of 75 mph enters a tunnel 3 ¹/₂ miles long. The train is ¹/₄ mile long. How long does it take for the train to pass through the tunnel from the moment the front enters to the moment the rear emerges?
a) 2.5 min
b) 3 min
c) 3.5 min
d) 3.5 min

Answer: b
Explanation:
$$\eqalign{ & {\text{Total}}\,{\text{distance}}\,{\text{covered}} \cr & = \left( {\frac{7}{2} + \frac{1}{4}} \right)\,{\text{miles}} \cr & = \frac{{15}}{4}\,{\text{miles}} \cr & {\text{Time}}\,{\text{taken}} = \left( {\frac{{15}}{{4 \times 75}}} \right)\,{\text{hrs}} \cr & = \frac{1}{{20}}\,{\text{hrs}} \cr & = \left( {\frac{1}{{20}} \times 60} \right)\,\min \cr & = 3\,\min \cr} $$