Total thumb drive sales=50×24=$1200, so he got 1675-1200=$475 for the keyboards. The keyboards are $49 each so he sold 475/49=9.69. This is not an exact number so he must have sold no more than 9 keyboards. If he’d sold 10 keyboards he would have got $490 ($15 more than he did get, making the total sales $1690 instead of $1675). 9 keyboards would sell for $441 (making the total sales $1641 instead of $1675), which leaves 475-441=$34 unaccounted for (perhaps it was commission, which would be 34/1641=2.1% approx).