Problem Solving

A furniture dealer purchased a desk for 150 dollars and then set the selling price equall to the purchase price plus a mark up that was 40 percent of the selling price. If the dealer sold the desk at the selling price, what was the amount of the dealers gross profit from the purchase and sale of the desk.

40
60
80
90
100

I dont know what is wrong with my equation

selling price - purchase price = profit

(150 + 0.4 S) - 150 = 0.4 S ...> this equation does not get me anywhere