Hualian Department Store held celebration activities. Notebooks that used to cost 1.6 yuan each, but now "buy 3 get 1 free", Mr. Li bought 20 notebooks, how much does it cost now?
华联百货店搞庆典活动．原来每本卖1.6元的笔记本，现在"买3送1"，李老师买了20本笔记本，现在只需花多少元？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<gadget id="calculator">15 * 1.6</gadget>
<output>24</output>

<result>24</result>