The first half of an hour the person drove by car at a speed of 100 km / h

The first half of an hour the person drove by car at a speed of 100 km / h, and the second – by train at a speed of 60 km / h. Find the average speed.

Given:

v1 = 100 km / h – the speed of a person on the first section of the path;

v2 = 60 km / h – the speed of a person on the third section of the path;

t1 = t2 = 0.5 hours – the time interval during which a person traveled sections of the path.

It is required to determine the average speed of a person Vav (km / h).

The length of the first section of the path:

S1 = t1 * v1 = 100 * 0.5 = 50 kilometers.

The length of the second section of the track:

S2 = t2 * v2 = 60 * 0.5 = 30 kilometers.

The entire distance traveled is equal to:

S = S1 + S2 = 50 + 30 = 80 kilometers.

Then the average speed is:

Vav = S / t = S / (t1 + t2) = 80/1 = 80 km / h.

Answer: the average speed of a person is 80 km / h.