Correct Answer - Option 3 : 40
Given:
A train A of length 2800 m crosses another train B of length 3500 m in 630/11 seconds. Train B cross a platform of length 700 m in 60 seconds
Formula:
If the lengths of two trains are a and b,
Time taken to cross each other when travelling in same direction = (a + b)/(va - vb)
Time taken to cross each other when travelling in opposite direction = (a + b)/(va + vb)
where va and vb are the speeds of trains A and B respectively.
Calculation:
Let the speeds of train A and B be a and b respectively.
(2800 + 3500)/(a + b) = 630/11 ----(1)
⇒ (3500 + 700)/b = 60
⇒ b = 4200/60 = 70
Hence, speed of B is 70m/s
Putting b = 70 in equation 1, we get,
(2800 + 3500)/(a + 70) = 630/11
⇒ a = 40
Therefore, speed of train A is 40 m/s.
