A store sells a kind of children's clothing. If each set is priced at 120 yuan, then 70% of the selling price is the purchase price, and 30% of the selling price is the money earned. Now we want to engage in promotional activities, in order to ensure that a set of children's clothing earns 18 yuan, how should we determine the discount?
商店卖一种童装，如果每套售价为120元，那么售价的70%是进价，售价的30%就是赚的钱．现在要搞促销活动，为保证一套童装赚18元钱，应该怎样确定折扣？
<result>18</result>