a) 5.6 sec
b) 7.2 sec
c) 8 sec
d) 9 sec.
Answer: c
Explanation:
$$\eqalign{
& {\text{Relative speed}} \cr
& {\text{ = (80 + 55)km/hr}} \cr
& {\text{ = 135 km/hr}} \cr
& {\text{ = }}\left( {135 \times \frac{5}{{18}}} \right)m/\sec \cr
& = \left( {\frac{{75}}{2}} \right)m/\sec \cr
& {\text{Distance covered}} = (120 + 90 + 90)m \cr
& {\text{ = 300m}} \cr
& {\text{Required time}} \cr
& {\text{ = }}\left( {300 \times \frac{2}{{75}}} \right)\sec \cr
& = 8\sec \cr} $$
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