A train is going at a speed of 60kmph towards Delhi and returned back at a speed of 30kmph. What is its average speed?
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Average Speed = (Total Distance/Time Taken);
Lets assume the distance traveled by train in one direction is x, and time taken while going towards delhi is t1, while it took t2 time while coming back from Delhi.
We can say
x/t1 = 60
x/t2 = 30
Total Average speed = 2x/(t1 + t2)
average speed = 2x /(x/60+x/30) = (2x/90x)*60*30 = 40
Find Number present only once