Option 4 : 40 sec

**Given:**

Lengths = 80m, 120 m

Speed ratio = 2 : 3

Crossing time = 8 sec

**Concept:**

The relative speed while running in opposite directions is the sum of individual speeds and while running in the same direction it is the difference of the individual speeds.

Distance traveled in crossing or overtaking of two trains is equal to the sum of lengths of both the trains.

**Formula used:**

Speed = distance/time

**Calculation:**

Let speeds be 2x and 3x

A.T.Q

(120 + 80)/(2x + 3x) = 8

⇒ 40/x = 8

⇒ x = 5

∴ Required time = 200/(15 - 10) = 40 sec

**∴ The time taken is 40 sec**