A bus moving at a speed of 45 km/hr catches a truck 150 metres ahead going in the same direction in 30 seconds. The speed of the truck is :

Answer: Option A

Let the speed of truck is = x km/hr
Their relative speed in same direction = (45 - x) km/hr
[Here (45 - x) has been written because bus crosses the truck which is running 150 metres ahead of it. i.e., Truck speed will be lower than that of bus]
According to the question,
$$\begin{array}{rl}& \text{Time}=\frac{\text{Distance}}{\text{Speed}}\\ & \Rightarrow \frac{150}{\left(45-x\right)\times \frac{5}{18}}=30\\ & \Rightarrow \frac{150\times 18}{\left(45-x\right)\times 5}=30\\ & \Rightarrow x=27\text{ km/hr}\end{array}$$