Answer
23 pieces sold at 9.50, 67 pieces sold at 7.50
Work Step by Step
$x$ pieces at 9.50
$90-x$ pieces at 7.50
90 pieces for total of 721
$9.5*x+(90-x)*7.5=721$
$9.5x+675-7.5x=721$
$2x+675=721$
$2x = 46$
$2x/2 =46/2$
$x=23$
$90-23=67$