Question 536730
The student played a total of 500+x games of which he won 370+x.  This ratio is given as 75% or 0.75, so...
{{{(370+x)/(500+x) = 0.75}}}  Cross-multiply.
{{{370+x = 0.75(500+x)}}}
{{{370+x = 375+0.75x}}} Subtract 0.75x from both sides.
{{{370+0.25x = 375}}} Subtract 370 from both sides.
{{{0.25x = 5}}} Finally, divide both sides by 0.25
{{{highlight(x = 20)}}}