Question

If Car A is moving 10 m/s east and Car B is moving 15 m/s west.

Answer 1

Car A is moving at a speed of 25 m/s east relative to Car B.

Let O denote an observer. Then

$v_{A/B}=v_{A/O}+v_{O/B}=v_{A/O}-v_{B/O}$

where $v_{X/Y}$ denotes the velocity of a body X relative to another body Y. We're told that

$v_{A/O}=10\dfrac{\rm m}{\rm s}$

and

$v_{B/O}=-15\dfrac{\rm m}{\rm s}$

where we take east to be to the positive direction. Then

$v_{A/B}=(10-(-15))\dfrac{\rm m}{\rm s}=25\dfrac{\rm m}{\rm s}$