The speed of the train going from Nagpur to Allahabad is 100 km/h while when coming back from Allahabad to Nagpur, its speed is 150 km/h. find the average speed during whole journey.

A. 125 km/hr

B. 75 km/hr

C. 135 km/hr

D. 120 km/hr

$\begin{array}{l} \text { Average speed, }\\ =\Large\frac{2 \times x \times y}{x+y}\\ =\Large\frac{2 \times 100 \times 150}{100+150}\\ =\Large\frac{200 \times 150}{250}\\ =120 \mathrm{~km} / \mathrm{hr} \end{array}$