SOLUTION: Why the speed of one train with respect to other train is twice when they are running parallel to each other but in opposite directions

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Question 1000249: Why the speed of one train with respect to other train is twice when they are running parallel to each other but in opposite directions
Answer by josgarithmetic(39630) About Me  (Show Source):
You can put this solution on YOUR website!
Opposite direction restricting to one dimension, and usage of signed numbers.

If both trains SPEEDS are each r, 

WhichWay
DIRECTION     r
OPPOSITE     -r


These will have their own sizes on a number line, find the difference in RATE (not speed, but rate; now you are dealing with the meaning of direction, not just size alone).

r-%28-r%29
r%2Br
2r