A car covered 150 km in 5 hours. If it travels at one-third its usual speed, then how much more time will it take to cover the same distance?

Given, the car covered 150 km in 5 hours

Speed of the car = $$\frac{150}{5}$$ = 30 km/h

One third of the speed = $$\frac{1}{3}\times30$$ = 10 km/h

Time required for the car to cover 150 km with one third speed = $$\frac{\text{Distance}\\ }{\text{Speed}}$$ = $$\frac{150}{10}$$ = 15 hours

$$\therefore\ $$Extra time required to cover the distance with one third speed = 15 - 5 = 10 hours

Hence, the correct answer is Option C