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## Homework Statement

Two rockets A and B are moving away from the earth in opposite directions at 0.85c and -0.75c respectively.

How fast does A measure B to be travelling?

Now I have worked out v = -0.85-0.75/(1- -0.85*-0.75) = -0.997. This is correct.

Now I would like to work it out backwards to check my understanding so:

According to A:

B travels at 0.997c

The earth frame travels at 0.75c

Q - How fast does the earth measure B to be travelling?

## Homework Equations

w=u-v/(1-uv)

## The Attempt at a Solution

I expected the answer to be 0.85c but:

v=0.997-0.75/(1- 0.997*0.75) == 0.979 and not 0.85.

Thanks for any help