well, for that one week, he made his salary, 200 bucks, plus 15% of his sales, because his sales were over $1000, to be exact, they were $2500, so he made 15% of 2500 extra on top of his salary.
so, what's 15% of 2500? well, is just (15/100) * 2500.
so, add that to his $200 salary, and that's his gross pay.