If a car travels a distance with 20% less speed, then it will reach 15 minutes late. What is the usual time (in minutes) taken by the car to travel the same distance?

Let usual speed of car = $$s_1=5$$ km/hr and usual time taken = $$t_1=t$$ hours

New speed = $$s_2=5-(\frac{20}{100}\times5)$$

= $$5-1=4$$ km/hr

New time taken = $$t_2=(t+\frac{15}{60})$$ hours

$$\because speed\propto\frac{1}{time}$$ => $$\frac{s_1}{s_2}=\frac{t_2}{t_1}$$

=> $$\frac{5}{4}=\frac{t+\frac{1}{4}}{t}$$

=> $$5t=4t+1$$

=> $$5t-4t=1$$

=> $$t=1$$ hour

$$\therefore$$ Usual time taken by the car to travel the same distance = 1 hour = 60 minutes

=> Ans - (D)