A train travels from A to B with speed 20 km/hrs and from B to A with speed 30 km/hrs. Average sp

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09 Apr 2010 Train Puzzle

A train travels from A to B with speed 20 km/hrs and from B to A with speed 30 km/hrs.
Average speed (in km/hrs) of the train for the whole journey is

OPtion

1) 21
2) 25
3) 24
4) 27
5) 26.5
6) 24.66
7) 25.66
8) 28
9) 29
10) 23.5

Solution

Let the distance between A to B is L km.
Total distance covered = 2L km.
Time taken in the journey = (L/20) + (L/30) = 50L/600 = L/12
Hence average speed = 2L/(L/12) = 24 km/hrs

Correct Option: 3 Best Solution (12)

Karthik Pandiyan   14 years AGO

Avg Speed =2xy/x+y
here x= speed of train from A to B=20 km/hrs
y= speed of the train from B to A = 30 km/hrs

substitute in above eqn we get avg.speed of the train = 2(20)(30)/(20+30)
= 120/50
=24

Like? Yes (1) | No   

ASHOK ROY   14 years AGO

The time required to travel distance x km = x/20 and x/30 in both cases . So the total time required x/30 + x/20 = x/12 . So the average speed to cover distance 2x will be = 2x/(x/12) = 24 kms

Like? Yes (1) | No   

sanjib biswas   14 years AGO

let total distence 60 km then time tken 3+2 =5 hrs ande bothe side distence 120 and avg. speed of whole jurney 24km/hr

Like? Yes (1) | No   

Dr.Dipin Singh   14 years AGO

Let Distance AB be 60 kms.

Then time taken from A to B is 3 hrs and Time taken from B to A is 2 hrs.

Total time taken for 120 kms is 5 hrs.

so average speed = 120/5 = 24 Km /hr

Like? Yes (1) | No   

karan    14 years AGO

used time speed formula for average speed..of forward and return trip

Like? Yes (1) | No   

SUDESHNA SEN   14 years AGO

Let the distance between A and B be x km. If time taken to travel from A to B is t1, then
t1= x/20, and if time taken to travel from B to A is t2, then t2= x/30.
Then total time taken= t1+t2= (x/20)+(x/30)= (x/12) and total distance travelled= x+x= 2x.
Then average speed of the whole journey= (Total distance travelled)/ (Total time taken)= (2x)/(x/12)= 24 km/hrs......Option 3)

Like? Yes (1) | No   

Dolly   14 years AGO

2*30*20/(20+30)= 24

Like? Yes (1) | No   

Bimal   14 years AGO

2*20*30/(20+30) = 24 kmph

Like? Yes (1) | No   

Dimple   14 years AGO

Suppose Distance from A to B be 20*30 =600 kms.

Then time taken from A to B is 30 hrs and Time taken from B to A is 20 hrs.

Total time taken for 1200 kms is 20+30 =50 hrs.

Then average speed for complete 1200 km journey = 1200/50 = 24 Km /hr

Like? Yes (1) | No   

~Sandeep~   14 years AGO

2x/{(x/20)+(x/30)} = 24

Like? Yes (1) | No   

Garima   14 years AGO

20*30*2/50=24

Like? Yes (1) | No   

shailender   14 years AGO

average speed=2d/((d/20)+(d/30))

Like? Yes (1) | No