The following question is accompanied by three statements 1, 2 and 3. You have to determine which statement (s) is/ are sufficient necessary to answer the questions.

What is the relative speed of two trains running in opposite directions?

1. They take 15 seconds to cross each other.

2. The speed of one of the trains is 60 Kmph.

3. The total length of the trains is 360 m.

In this case, the "distance" is the total length of both the trains, and the "time" is the time taken for them to cross each other. In terms of the trains' speeds, the relative speed of two objects moving in opposite directions is the sum of their individual speeds.

Given the three statements:

They take 15 seconds to cross each other.
The speed of one of the trains is 60 Kmph.
The total length of the trains is 360 m.
We can see that with statements 1 and 3 together, the relative speed of both trains can be found, since we have the total length (distance) the trains need to cross and the time taken for them to cross each other. Statement 2 only gives the speed of one train, which isn't sufficient to find their combined relative speed.

So, (3) Only 1 and 3 together are sufficient to find the relative speed of the two trains.