A man travels 39 km from A to B at 15 km/hr and then 52 km from B to C at 18 km/hr. What is his average speed (in km/hr) for the whole journey?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(16\frac{11}{19}\)
Average speed is total distance divided by total time, not the average of the two speeds.
Total distance \(=39+52=91\) km
Time from A to B \(\displaystyle =\frac{39}{15}=\frac{13}{5}\) hr
Time from B to C \(\displaystyle =\frac{52}{18}=\frac{26}{9}\) hr
Total time \(\displaystyle =\frac{13}{5}+\frac{26}{9}=\frac{117+130}{45}=\frac{247}{45}\) hr
Average speed \(\displaystyle =91\times\frac{45}{247}=\frac{7\times45}{19}=\frac{315}{19}=16\frac{11}{19}\) km/hr