Xiaohong writes 21 big characters in 3 minutes, and Xiao Ming writes 63 big characters in 7 minutes. How much less can Xiaohong write than Xiaoming per minute.
小红3分钟写21个大字，小明7分钟写63个大字，小红每分钟比小明少写多少个．
<gadget id="calculator">63 / 7</gadget>
<output>9</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">9 - 7</gadget>
<output>2</output>

<result>2</result>