A train is going at a speed of 60 kmph towards Delhi and returned back at a speed of 30 kmph. What is its average speed?
a) 10
b) 20
c) 30
d) 40
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A train travels with a speed of 30km/h and returns with a speed of 50km/h. What is the the average speed of the train: (a) 36kmh, (b) 37.5 kmh, (c) 38 kmh, or (d) 40 kmh?
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Sid Kemp, Grandson of a physicist, and an author and engineer who loves explaining things.
Answered Dec 2, 2016
We can assume in this word problem that the train travels in one direction from a location we’ll call Point A to another location we will call Point B. It then returns along the same route in the opposite direction from Point B to Point A.
We don’t know the distance travelled, so let’s call the distance between Point A and Point B D for distance. The total distance is then 2D.
The average speed is:
average speed = (speed for the outbound trip + speed of the return trip) / 2 = (30 + 50) / 2 = 80 / 2 = 40 km/hour. The correct multiple choice answer is (d).
This works even if we don’t know the distance traveled because we know that half the distance was traveled at one speed, and the other, equal half was traveled at the other speed. So the unknown variable, D, for distance, cancels out and does not matter.
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Minnanainathan Kripasankar, Project Engineer at Wipro Technologies (2015-present)
Answered May 13
You can use below methods instead of traditional speed = Distance/time method
Method 1:
S1 - speed 1
S2 - speed 2
Considering distance same.
Average speed = 2*S1*S2/(S1+S2)
Average speed = 2*30*50/(30+50)
= 3000/80=37.5
Method 2:
Making ratio 30:50 = 3:5
Difference between speeds = 50 - 30 = 20
Dividing difference by sum of ratios = 20 /8 = 2.5
Taking lowest speed and lowest of ratio
Average speed = 30 + 3 * 2.5 = 37.5
hope it helps y
This question previously had details. They are now in a comment.
Answer
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10 ANSWERS

Sid Kemp, Grandson of a physicist, and an author and engineer who loves explaining things.
Answered Dec 2, 2016
We can assume in this word problem that the train travels in one direction from a location we’ll call Point A to another location we will call Point B. It then returns along the same route in the opposite direction from Point B to Point A.
We don’t know the distance travelled, so let’s call the distance between Point A and Point B D for distance. The total distance is then 2D.
The average speed is:
average speed = (speed for the outbound trip + speed of the return trip) / 2 = (30 + 50) / 2 = 80 / 2 = 40 km/hour. The correct multiple choice answer is (d).
This works even if we don’t know the distance traveled because we know that half the distance was traveled at one speed, and the other, equal half was traveled at the other speed. So the unknown variable, D, for distance, cancels out and does not matter.
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Minnanainathan Kripasankar, Project Engineer at Wipro Technologies (2015-present)
Answered May 13
You can use below methods instead of traditional speed = Distance/time method
Method 1:
S1 - speed 1
S2 - speed 2
Considering distance same.
Average speed = 2*S1*S2/(S1+S2)
Average speed = 2*30*50/(30+50)
= 3000/80=37.5
Method 2:
Making ratio 30:50 = 3:5
Difference between speeds = 50 - 30 = 20
Dividing difference by sum of ratios = 20 /8 = 2.5
Taking lowest speed and lowest of ratio
Average speed = 30 + 3 * 2.5 = 37.5
hope it helps y
Answered by
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THE SHORTCUT TO FIND THE ANS IS:-
2*S1*S2\S1+S2
2*60*30/60+30
=40
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