A billboard in a supermarket reads: Bright Milk 2.5 yuan/bottle, 1 free bottle for full 4 bottles, 2 free bottles for full 8 bottles, no cap. Mom wants to buy 10 bottles of milk, how much does she only need to pay? How much is it actually discounted?
某超市广告牌上写着：光明牛奶2.5元/瓶，满4瓶送1瓶，满8瓶送2瓶，上不封顶．妈妈要买10瓶牛奶，只要付多少元？实际上打了几折？
<gadget id="calculator">2.5 * 8</gadget>
<output>20</output>

<result>20</result>