Find the average speed of the skier if he covered the first 10 km in 12 minutes, the next 20 km in 28 minutes.

Given:

S1 = 10 kilometers = 10,000 meters – the length of the first section of the route;

t1 = 12 minutes = 720 seconds – the time during which the skier covered the first section of the path;

S2 = 20 kilometers = 20,000 meters – the length of the second section of the route;

t2 = 28 minutes = 1680 seconds – the time during which the skier covered the second section of the path.

It is required to determine v (m / s) – the average speed of the skier along the entire path.

The entire path traversed by the skier is:

S = S1 + S2 = 10,000 + 20,000 = 30,000 meters.

The time the skier was in motion is:

t = t1 + t2 = 720 + 1680 = 2400 seconds.

Then, the average speed is:

v = S / t = 30000/2400 = 12.5 m / s.

Answer: The average speed of a skier is 12.5 m / s (45 km / h).