If a car went the first third of the distance at 10 kmh, the second third at 24 kmh, and the last third at 48 kmh, what was the average speed of the car for the entire trip?  Options: A) 36 kmh B) 19 kmh C) 42 kmh D) 44 kmh E) 50 kmh

Rationale: "assume d / 3 = 240 ( this number is convenient because it is divisible by 10 , 24 and 48 ) so : 240 = 10 * t 1 = 24 hrs 240 = 24 * t 2 = 10 hrs 240 = 48 * t 3 = 5 hrs t = t 1 + t 2 + t 3 = 39 hrs d = rt ( 240 * 3 ) = r * 39 r = 19 answer : b"
Answer: B