Amit goes to his office by car at the speed of 80 km\hr and reaches 15 minutes earlier. If he goes at the speed 60 km/hr, he reaches 15 minutes late. What will be the speed (in km/hr) of the car to reach on time ?

Let ideal time taken to reach on time = $$t$$ hours

Speed is inversely proportional to time

=> $$\frac{80}{60}=\frac{t+\frac{1}{4}}{t-\frac{1}{4}}$$

=> $$80t-20=60t+15$$

=> $$80t-60t=20t=15+20$$

=> $$t=\frac{35}{20}=\frac{7}{4}$$ hours

Thus, distance covered by going at 60 km/hr and reaching in $$(\frac{7}{4}+\frac{1}{4}=2)$$ hours = $$60\times2=120$$ km

$$\therefore$$ Ideal speed to reach on time = $$\frac{120\times4}{7}=68\frac{4}{7}$$ km/hr

=> Ans - (C)