The shopping mall held a promotion on the National Day. A fountain pen cost 24 yuan. Buy 5 pens and get 1 free. Xiaohong bought 6 pens. How much is each pen cheaper on average?
国庆节商场搞促销活动，钢笔24元1支，买5支送1支，小红买了6支，平均每支便宜了多少钱？
<gadget id="calculator">5 / 6</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">24 * (5/6)</gadget>
<output>20</output>

<gadget id="calculator">24 - 20</gadget>
<output>4</output>

<result>4</result>