P travels in his car and covers 2/5 part of his journey at a speed of 20 km/h, 1/8 part at 5 km/h and the remaining 19/40 part at 19 km/h. What is the average speed of the car during the entire journey?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 14.3 km/h
Average speed is total distance divided by total time, not the average of the three speeds, because the car spends different times at each speed.
Assuming the whole journey to be 40 km:
\(\frac{2}{5}\) of 40 = 16 km at 20 km/h: time \(=16\div20=0.8\) h
\(\frac{1}{8}\) of 40 = 5 km at 5 km/h: time \(=1\) h
\(\frac{19}{40}\) of 40 = 19 km at 19 km/h: time \(=1\) h
Total time \(=0.8+1+1=2.8\) h
Average speed \(\displaystyle =\frac{40}{2.8}=14.29\approx14.3\) km/h