Correct Answer - Option 4 : 720/43 km per hr
Boy 1/3 distance walked at speed = 10 km per hr
Next 1/4 distance walked at speed = 20 km per hr
Rest distance walked at speed = 30 km per hr
Here, the man covers not equal distance at different speeds.
Let the distance be x km.
∴ Time is taken at 10 km per hr. distance one third (t1) = x/(3×10) hrs
Time is taken at 20 km per hr. distance one-fourth (t2) = x/(4×20)
Rest journey = x − (x/3 + x/4)
Rest journey time is taken at 30 km per hr. (t3) = 5x/(12×30) hrs
Total time is taken in the whole journey = x/30 + x/80 + x/72
⇒ (24x + 9x + 10x)/720
Average speed is = (x)/43x/720
⇒ 720/43 km per hr