As a smart consumer, you are always on the lookout for sales, coupons, and rebates. While shopping for new clothes, you notice that one particular 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.