Two trains of equal length when moving in opposite direction takes 5 sec. to cross each other completely and when moving in same direction takes 15 sec. Find the ratio of their speed.

One Answer on this Question

1. Let’s assume the length of each train is ‘L’ and the speeds of the two trains are ‘V₁’ and ‘V₂’ respectively.

   When the trains are moving in the opposite direction, their relative speed is the sum of their individual speeds. The total distance they need to cover is the sum of their lengths. Since they cross each other completely in 5 seconds, we can set up the following equation:

   (V₁ + V₂) × 5 = 2L

   When the trains are moving in the same direction, their relative speed is the difference between their individual speeds. The total distance they need to cover is the difference between their lengths. Since they cross each other completely in 15 seconds, we can set up the following equation:

   (V₁ – V₂) × 15 = 2L

   Now, let’s solve these equations to find the ratio of their speeds.

   From the first equation, we have:
   (V₁ + V₂) × 5 = 2L
   V₁ + V₂ = (2L) / 5

   From the second equation, we have:
   (V₁ – V₂) × 15 = 2L
   V₁ – V₂ = (2L) / 15

   Let’s add these two equations together:
   V₁ + V₂ + V₁ – V₂ = (2L) / 5 + (2L) / 15
   2V₁ = (6L + 2L) / 15
   2V₁ = (8L) / 15
   V₁ = (4L) / 15

   So, the speed of the first train is (4L) / 15.

   Now, let’s substitute this value back into the first equation to find V₂:
   (4L) / 15 + V₂ = (2L) / 5
   V₂ = (2L) / 5 – (4L) / 15
   V₂ = (6L – 4L) / 15
   V₂ = (2L) / 15

   Therefore, the speed of the second train is (2L) / 15.

   The ratio of their speeds is given by:
   (V₁ / V₂) = ((4L) / 15) / ((2L) / 15)
   (V₁ / V₂) = 4L / 2L
   (V₁ / V₂) = 2

   So, the ratio of their speeds is 2:1.

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