a) 120 m
b) 130 m
c) 140 m
d) 150 m
Answer: b
Explanation: Length of train
$$ = \frac{{{\text{Length}}\,{\text{of}}\,{\text{the}}\,{\text{platform}}}}{{{\text{Difference}}\,{\text{in time}}}}$$ × (Time taken to cross a lamp post)
$$\eqalign{
& = \frac{{390}}{{28 – 7}} \times 7 \cr
& = \frac{{390}}{{21}} \times 7 \cr
& = \frac{{390}}{3} \cr
& = 130\,{\text{m}} \cr} $$
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