Customers can use a manufacturer's coupon and a store coupon to obtain a discount when buying soap powder in a certain store. In one week, 70 percent of the customers used the store coupon and 35 percent used the manufacturer's coupon when purchasing the soap powder. 15 percent of the customers used neither coupon when purchasing the soap powder. What percent of the customers who used a coupon used both the manufacturer's coupon and the store coupon?