Two trains of lengths 120 m and 90 m are running with speeds of 80 km/hr and 55 km/hr, respectively, towards each other on parallel lines. If they are 90 m apart, after how many seconds they will cross each other completely?

The speeds of the two trains in m/s would be $$80\times\ \frac{5}{18}=22.22$$ m/s and $$55\times\ \frac{5}{18}=15.277$$ m/s
Giving their relative speed to be 37.5 m/s

The total distance both trains essentially have to cover would be the distance between them + length of train 1 + length of train 2
Distance to be travelled = 90+120+90 = 300m

The time taken would then be $$\frac{300}{37.5}=8$$ second

Therefore, Option C is the correct answer.