Xiao Ming and Xiao Li competed in writing. Xiao Ming wrote 90 characters in 5 minutes, and Xiao Li wrote 84 characters in 4 minutes. Which one of them could write faster? How much faster per minute?
小明和小丽比赛写字，小明5分钟写90个字，小丽4分钟写84个字，他们谁写得快？每分快多少？
<gadget id="calculator">84 / 4</gadget>
<output>21</output>

<gadget id="calculator">90 / 5</gadget>
<output>18</output>

<gadget id="calculator">21 - 18</gadget>
<output>3</output>

<result>3</result>