A train A is running in direction opposite to a train B. The speed of ...
Given data:
- Speed of train A = 72 kmph
- Time taken by train A to cross a pole = 12 seconds
- Speed of train B = 54 kmph
- Time taken by train B to cross a platform of length 120 m = 112/3 seconds
To find: Time taken by train A to cross train B
Approach:
1. First, we need to find the length of train A or train B. Let's assume the length of train A is L1.
2. Using the formula, distance = speed × time, we can calculate the length of train A as follows:
- Distance covered by train A in 12 seconds = L1
- Speed of train A = 72 kmph = 20 m/s (1 kmph = 5/18 m/s)
- Therefore, L1 = 20 × 12 = 240 m
3. Now, let's find the length of train B. Let's assume the length of train B is L2.
4. Using the formula, distance = speed × time, we can calculate the length of train B as follows:
- Distance covered by train B in 112/3 seconds = Length of platform + Length of train B
- Speed of train B = 54 kmph = 15 m/s (1 kmph = 5/18 m/s)
- Length of platform = 120 m
- Therefore, (120 + L2) = 15 × 112/3 = 560 m (distance covered by train B in 112/3 seconds)
- Therefore, L2 = 440 m
5. Let's assume that both trains meet after time 't'.
6. Using the formula, distance = speed × time, we can write the following equation:
- Distance covered by train A in time 't' = Distance covered by train B in time 't' + L2
- Speed of train A = 72 kmph = 20 m/s
- Speed of train B = 54 kmph = 15 m/s
- Distance covered by train A in time 't' = 20t
- Distance covered by train B in time 't' = 15t
- Therefore, 20t = 15t + 440 (adding L2 on both sides)
- Therefore, t = 88/5 seconds = 17.6 seconds (dividing both sides by 5)
Therefore, the approximate time taken by train A to cross train B is 17.6 seconds, which is closest to option B (19 seconds).