The distance between the two cities is 135 km. The car drove 45 km at a speed of 90 km / h.

The distance between the two cities is 135 km. The car drove 45 km at a speed of 90 km / h. Then it rained heavily, and on a slippery road the speed had to be reduced to 60 km / h. Determine the average speed of the car along the way.

S = 135 km.

S1 = 45 km.

V1 = 90 km / h.

V2 = 60 km / h.

Vav -?

To find the average vehicle speed Vav, it is necessary to divide the entire path S it has traveled by the time of its movement t: Vav = S / t.

The travel time of the entire path is expressed by the sum: t = t1 + t2, where t1 is the time of movement on the first part of the path, t2 is the time of movement on the second part of the path.

Since the car moved uniformly on the sections, we express the time of movement by the formulas t1 = S1 / V1, t2 = S2 / V2.

t1 = 45 km / 90 km / h = 0.5 h.

S2 = S – S1.

S2 = 135 km – 45 km = 90 km.

t2 = 90 km / 60 km / h = 1.5 h.

Vav = 135 km / (0.5 h + 1.5 h) = 67.5 km / h.

Answer: the average vehicle speed is Vav = 67.5 km / h.