Question

Anna, ben, and cindy have m markers together. anna has a markers, and this is twice as many markers as ben has. how many markers does cindy have?

Answer 1

So, Anna has a markers, Ben has b markers and Cindy has c markers. 
We know that a+b+c = m.
And we know also that a = 2b. ( twice as many markers as ben has)
(Remember b is the number of Ben markers.)
The equation becomes:
2b+b+c=m.
Solving for c:
$c=m-3b$
Cindy has (m-3b) markers.