Xiao Ming: I buy 5 pens for 15 yuan. Xiaohong: The pens I bought are 4 yuan each. Who bought the cheapest pens, and how much is each cheaper?
小明：我买5支笔15元．小红：我买的笔每支4元．谁买的笔便宜，每支便宜多少钱？
<gadget id="calculator">15 / 5</gadget>
<output>3</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<result>1</result>