M11-03

This is the concept of Relative Speed.

When two objects (speeds V1 and V2) move in opposite directions (towards each other or away from each other), they cover the distance between them (or create distance between them) at the rate of (V1 + V2).

When two objects move in same direction, their speeds get subtracted.

I think this is a high-quality question and I agree with explanation.

I still did not get this part:

V+40=90V

WHY IS IT SO?

Maybe my reply is outdated to you, but it could be relevant for those who face this question in the future.
Firstly, to understand this question and the solution you need to be good with the concept of relative speed.
What is relative speed? Basically, it is speed of an object relative to another object. For example- Person A is standing still and B is moving at a speed of 5 mph. What is their relative speed to each other? That is right, 5 mph! But if they started to move to opposite directions: A moves at 2 mph and B at 5 mph in opposite directions, they moving away from each other at 7 mph, therefore they relative speed is 7 mph. If they started to move toward each other: A at 2 mph, and B at 5 mph, they are still covering 7 mph. If they are moving in the same direction, say, A at 2 mph and B at 5 mph, they relative speed to each other is 3 mph. Hope it makes sense.
Back to your question, this equation is basically=(unknown speed at kmph)+(known speed at kmph)=(relative speed toward each other at kmph)
Hope it helps

I think this is a high-quality question and I agree with explanation.

I have edited the question and the solution by adding more details to enhance its clarity. I hope it is now easier to understand.