A bus is travelling the first one-third distance at a speed of 10 km/h, the next one-fourth at 20 km/h and the remaining at 40 km/h. What is the average speed of the bus?
Let the total distance be 100 km
One third of 100 km = 33.33 km
time = 
t1 = 
= 3.33 hr
Distance left = 100 km – 33.33 km = 66.67 km
One fourth of 66.67 km = 16.67 km
t2 = 
= 0.83 hr
Remaining distance = 100 km – 33.33 km – 16.67 km = 50 km
t3 = 
= 1.25 hr
t1+t2+t3 = 5.41 hr
Average speed = 
= 
= 18.4 km/hr
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