P6 Maths - Speed

Mr Tan sends his son to school every morning at 6.15am. If he drives at an average speed of 60km/h, his son will be 10 min late. If he speeds at 120km/h, his son will be 10 min early. What speed should he drive at to arrive on exactly on time?

When the distance covered is the same, the ratio of time taken is opposite to the ratio of the speed.

Speed ratio = 60 : 120 = 1 : 2
Time ratio = 2 : 1

Difference = 2 - 1 = 1 unit

(Difference between 10 min early and 10 min late = 20 min)



1 unit --> 20 min
2 units --> 40 min

When he travels at 60 km/h, he will take 40 min, or 2/3 hr.

Distance to school = 60km * 2/3h
= 40 km

To be on time, the time taken is 30 min, (10 min less than 40min)

Hence Speed = 40km / 1/2h
= 80km/h.