An online store on Taobao held a "June 1st" promotion, and all four notebooks will get one free. A kind of notebook is 5 yuan each, and the owner of the stationery store buys 100 copies, how much does it cost?
淘宝某网店举办"六一"促销活动，所有笔记本四本送一本．一种笔记本每本5元，文具店老板买100本，花了多少元？
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">5 * (4/5) * 100</gadget>
<output>400</output>

<result>400</result>