Question 135983
Let's change 0.75 to 3/4 first. 
{{{3/4 = a/(a+v)}}}
To get rid of the denominators, multiply both sides by 4(a+v).
{{{cross(4)(a+v)3/cross(4)=4cross((a+v))a/cross((a+v))}}}
{{{3(a+v)=4a)}}} Simplify.
{{{3a+3v=4a)}}} Distribute.
{{{3a-3a+3v=4a-3a}}} Subtract 3a from both sides.
{{{3v=a}}} Simplfy.
{{{v=a/3}}} Divide both sides by 3.