To better give a visual, I drew it out instead.
The attachment is the solution.
Q - r = r
Flip
r = Q - r
Add r to both sides
2r = Q
Divide both sides by 2
r = Q/2
Have an awesome day! :)
A direct proportional variation, is really just a slope-intercept form of equation, except that the y-intercept is always 0, and the constant of variation "k" is the slope pretty much.