A car completed a journey of 400 km in 121/2 hrs. The first 3/4 of the journey was done at 30 km/hr. Calculate the speed for the rest of the journey.

Answer: Option B

$$\begin{array}{rl}& \text{Total journey = 400}\\ & \frac{3}{4}\text{ Journey = 400}\times \frac{3}{4}\\ & =300\text{ km}\end{array}$$
Remaining journey = 100 km
Let the speed of car for the rest of journey = x km/hr
According to the question,
$$\begin{array}{rl}& \frac{300}{30}+\frac{100}{x}=12\frac{1}{2}\\ & 10+\frac{100}{x}=\frac{25}{2}\\ & \frac{100}{x}=\frac{25-20}{2}\\ & x=\frac{100\times 2}{5}\\ & x=40\text{ km/hr}\end{array}$$