There is a mechanical pencil in a stationery store, which costs 20 yuan for every 5 sticks when purchased, and 20 yuan for every 4 sticks when sold.
文具店有一种活动铅笔，进货时每5枝20元，出售时每4枝20元，店主要想赚得50元钱，需卖出这种活动铅笔多少枝．
<gadget id="calculator">20 / 4</gadget>
<output>5</output>

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

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">50 / 1</gadget>
<output>50</output>

<result>50</result>