Two supermarkets are promoting a gel pen with a unit price of 3 yuan at the same time. Supermarket A buys four and gets one free, while supermarket B offers a 15% discount. Xiao Ming wants to buy 10 of these gel pens. Which supermarket should he buy? What if he is going to buy 7?
两家超市同时对一种单价为3元的中性笔搞促销活动，甲超市买四赠一，乙超市打八五折，小明要买10支这样的中性笔，到哪家超市买合适？如果他准备买7支呢？
<result>7</result>