A cyclist rides a bicycle 8 km at an average speed of 10 km/hr and again travels 10 km at an average speed of 8 km/hr. What is the average speed for the entire trip?
Choices
A) 9.28
B) 8.48
C) 8.78
D) 8.98
E) 9.18

Rationale: "distance = 18 km time = 8 / 10 + 10 / 8 = ( 64 + 100 ) / 80 = 164 / 80 = 41 / 20 hours average speed = ( 18 * 20 ) / 41 = 8.78 km / h the answer is c ."
Answer: C