A car was driving at 72 Km/h for 20 minutes, and then at 81 Km/h for another 40 minutes. What was its average speed?  Pick one
A) 76 .
B) 77 .
C) 78 .
D) 79 .
E) 80 .

Rationale: driving at 72 km / h for 20 minutes , distance covered = 72 * 1 / 3 = 24 km driving at 90 km / h for 40 minutes , distance covered = 81 * 2 / 3 = 54 km average speed = total distance / total time = 78 / 1 = 78 km / h answer : a
Answer: A