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TIME AND DISTANCE PROBLEMS

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TIME AND DISTANCE -> IMPORTANT FACTS AND FORMULAE

1. Speed = [Distance/Time],
Time=[Distance/Speed],
Distance = (Speed*Time)
2. x km/hr = [x*5/18] m/sec.
3. If the ratio of the speeds of A and B is a:b, then the ratio of the times taken by them to cover the same distance is 1/a : 1/b or b:a.
4. x m/sec = [x*18/5] km/hr.
5. Suppose a man covers a certain distance at x km/hr and an equal distance at y km/hr. then, the average speed during the whole journey is [2xy/x+y] km/hr.

TIME AND DISTANCE -> EXAMPLES

1. How many minutes does John take to cover a distance of 400 m, if he runs at a speed of 20 km/hr?
  Sol. John’s speed = 20 km/hr
= [20*5/18] m/sec
= 50/9 m/sec.
2. While covering a distance of 24 km, a man noticed that after walking for 1 hour and 40 minute, the distance covered by him was 5/7 of the remaining distance. What was his speed in metres per second?
  Sol. Let the speed be x km/hr.
Then, distance covered in 1 hr: 40 min. i.e., 1 2/3 hrs = 5x/3 km.
Remaining distance = [24-5x/3] km.
5x/3 = 5/7[24-5x/3] 5x/3 = 5/7[72-5x/3] 7x = 72-5x
12x = 72 x = 6
Hence, speed = 6 km/hr = [6*5/18] m/sec = 5/3m/sec = 1 2/3 m/sec
3. If a man walks at athe rate of 5 kmph, he misses a train by 7 minutes. However, if he walks at the rate of 6 kmph, he reaches the station 5 minutes before the arrival of the train. Find the distance covered by him to reach the station.
  Sol. Let the required distance be x km.
Difference in the times taken at two speeds = 12 min = 1/5 hr.
x/5 - x/6 = 1/5 6x - 5x = 6 x = 6.
Hence, the required distance is 6 km.

TIME AND DISTANCE -> EXERCISE

1. A man takes 5 hours 45 min. In walking to a certain place and riding back. He would have gained 2 hours by riding both ways. The time he would take to walk both ways is :
 
  • A. 11 hrs 45 min
  • B. 3 hrs 45 min
  • C. 7 hrs 30 min
  • D. 7 hrs 45 min
Ans: A.
Sol.
Let the distance be x km. Then,
(time taken to walk x km) + (Time taken to ride x km) = 23/4 hrs.
(Time taken to walk 2x km) + (Tie taken to ride 2x km) = 23/2 hrs.
But, time taken to ride 2x km = 15/4 hrs. ∴ Time taken to walk 2x km [23/2 - 15/4] hrs = 31/4 hrs = 7 hrs 45 min.
 
2. Two trains start from P and Q respectively and travel towards each other at a speed of 50 km/hr and 40 km/hr respectively. By the time they meet, the first train has travelled 100 km more than the second. The distance between P and Q is :
 
  • A. 500 km
  • B. 630 km
  • C. 900 km
  • D. 660 km
Ans: C.
Sol.
At the time of meeting, let the distane travelled byb the second train be x km. Then,
distance covered by the first train is (x + 100) km
x/40 = (x+100)/50 50x = 40x 4000 x = 400.
So, distance between P and Q = (x+x+100) km = 900 km.
 
 
3. Two trains starting at the same time from two stations 200 km apart and going in opposite directions cross each other at a distance of 110 km from one of the stations. What is the ratio of their speeds ?
 
  • A. 11:9
  • B. 11:20
  • C. 9:20
  • D. 11:8
Ans: A.
Sol.
In the same time, they cover 110 km and 90 km respectively.
∴ Ratio of their speeds = 110 : 90 = 11 : 9.