The train passed the first half of the time at a speed of 72 km / h, and the second half

The train passed the first half of the time at a speed of 72 km / h, and the second half of the time – 36 km / h. Determine the average speed.

Let the train pass the path S₁ in the first half of the time t / 2, moving at a speed v₁ = 72 km / h = 20 m / s, equal to:

S₁ = v₁ ∙ t / 2 or S₁ = 20 ∙ t / 2.

And in the second half of the time t / 2, moving at a speed v₂ = 36 km / h = 10 m / s, he passed:

S₂ = v₂ ∙ t / 2 or S₂ = 10 ∙ t / 2.

To determine the average train speed, we will use the formula:

v = (S₁ + S₂): (t₁ + t₂) or v = (20 ∙ t / 2 + 10 ∙ t / 2): (t / 2 + t / 2);

v = 15 m / s.

Answer: The average train speed is 15 m / s.