Answer:
Option A
Explanation:
Let the length of the train be $x$ m and its speed $y$ m/sec.
Distance covered in crossing the platform $=170+x$ m
Time taken = 21 seconds
Speed $y=\frac{(170+x)}{21}$ -----------------(1)
Distance covered in crossing the man $=x$ mts
Time Taken $=7\frac{1}{2}$ $=\frac{15}{2}$ sec
Speed $y=\frac{x}{\frac{15}{2}}$ $=\frac{2x}{15}$ ------------------------(2)
Eqating (1) and (2)
$\frac{(170+x)}{21}$ $=\frac{2x}{15}$
$x=\frac{850}{9}$ $=94\frac{4}{9}$
From (2) $y=\frac{2x}{15}$ $=\frac{(2\times 850)}{(9\times 15)}$ $=12\frac{16}{27}$