Question 495288
a boy travels with a speed of 30 kmph for first 1/3 journey time during the remaining journey time he travels with a speed 60kmph find the average speed of the boy
:
Let t = total time
then
{{{1/3}}}t = time at 30 km/h
and
{{{2/3}}}t = time at 60 km/h
:
Let a = average speed for the journey
:
Write a distance equation: dist = speed * time 
:
30km/h dist + 60 km/h dist = total dist
30*{{{1/3}}}t + 60*{{{2/3}}}t = at
We can cancel out the denominators
10t + 20(2t) = at
10t + 40t = at
we don't care about t, divide thru by t and eliminate it
10 + 40 = a
a = 50 km/h is the average speed