Xiaoxiao’s birthday is going to be on the weekend, so mother and Xiaoxiao go to the supermarket to buy yogurt. Lianhua Supermarket buys 3 boxes and gets 1 free, 2.3 yuan per box; Guangming Supermarket 1.9 yuan per box. Xiaoxiao wants to buy 15 boxes of yogurt, which supermarket do they go to buy a good deal? Why?
周末笑笑要过生日，妈妈和笑笑到超市买酸奶．联华超市买3盒送1盒，每盒2.3元；光明超市每盒1.9元．笑笑要买15盒酸奶，他们到哪个超市买划算？为什么？
<result>15</result>