a) 35 seconds
b) 36.5 seconds
c) $$\frac{{40}}{9}$$ seconds
d) 36 seconds
Answer: c
Explanation: Let length of each train be X m, then
$$\eqalign{
& {S_1} = \frac{X}{4} \cr
& {S_2} = \frac{X}{5} \cr
& {\text{Required time to cross each other,}} \cr
& = {\frac{{2X}}{{ { {\frac{X}{4}} + {\frac{X}{5}} } }}} \cr
& = \frac{{40}}{9}{\text{seconds}} \cr} $$
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