a) 120 m
b) 240 m
c) 300 m
d) None of these
Answer: b
Explanation:
$$\eqalign{
& {\text{Speed}} = {54 \times \frac{5}{{18}}} \,{\text{m/sec}} = 15\,{\text{m/sec}} \cr
& {\text{Length}}\,{\text{of}}\,{\text{the}}\,{\text{train}} = \left( {15 \times 20} \right){\text{m}} = 300\,{\text{m}} \cr
& {\text{Let}}\,{\text{the}}\,{\text{length}}\,{\text{of}}\,{\text{the}}\,{\text{platform}}\,{\text{be}}\,x\,{\text{metres}} \cr
& {\text{Then}},\,\frac{{x + 300}}{{36}} = 15 \cr
& x + 300 = 540 \cr
& x = 240\,{\text{m}} \cr} $$
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