As a smart consumer, you are always on the lookout for sales, coupons, and rebates. While shopping for new clothes, you notice that one brand of shirts is on sale for 20% off the original price. You also have a coupon for $10 off a $40 purchase. Coupons are applied after the 20% discount is taken. If each shirt originally costs $17.99, how many shirts would you need to buy to be able to use your coupon? Show your work.