The general answer is given by W=FX+CS where W=wages=500, F=fixed guaranteed part of the weekly wage, C is the commission as a fraction, and S is the value of sales over two weeks. X=2, the number of weeks. So 500=2F+CS; 500-2F=CS; S=(500-2F)/C.
EXAMPLE
C=15%, so C=0.15; F=100. S=(500-200)/0.15=300/0.15=30000/15=2000.