A train travels at a speed of 30 km/hr for 12 minutes and at a speed of 45 km/hr for the next 8 minutes. The average speed of the train for this journey is :

Answer: Option B

Total distance travelled :
$$\begin{array}{rl}& =\left(30\times \frac{12}{60}+45\times \frac{8}{60}\right)\text{ km}\\ & =\text{12 km}\end{array}$$
Total time taken :
= (12 + 8) min
= 20 min
= $$\frac{1}{3}$$ hr
∴ Average speed :
= (12 × 3) km/hr
= 36 km/hr