The first 9 km the bus traveled at a speed of 36 km / h, and the next 27 km at a speed of 54 km / h.

The first 9 km the bus traveled at a speed of 36 km / h, and the next 27 km at a speed of 54 km / h. What is the average speed of the bus.

Average speed:
Vav. = S / t, where S is the path traveled by the bus (S = S1 + S2, where S1 = 9 km, S2 = 27 km), t is the bus travel time (t = t1 + t2).
Time for a bus to pass two sections:
t1 = S1 / V1, t2 = S2 / V2, where V1 is the speed on the first leg (V1 = 36 km / h), V2 is the speed on the second leg (V2 = 54 km / h).
Vav. = S / t = (S1 + S2) / (S1 / V1 + S2 / V2) = (9 + 27) / (9/36 + 27/54) = 36 / 0.75 = 48 km / h.
Answer: The average bus speed is 48 km / h.