Naughty wants to write an 800-word manuscript, and has already written it for 15 minutes, at a rate of 20 words per minute. How many words are left?
淘气要演写一份800字的稿件，已经写了15分钟，每分钟写20个字，还剩多少字没写？
<gadget id="calculator">15 * 20</gadget>
<output>300</output>

<gadget id="calculator">800 - 300</gadget>
<output>500</output>

<result>500</result>