Xiao Ming loves calligraphy. He stipulates that he writes 30 characters per day on average every week (7 days). Write how many characters to meet the requirements.
小明爱好书法，他规定自己每周(7天)平均每天写30个大字，这个星期他前3天按时完成任务，周四因病没写，周五周六共写80个大字，周日他要写多少个大字才达到规定要求．
<gadget id="calculator">30 * 7</gadget>
<output>210</output>

<gadget id="calculator">30 * 3</gadget>
<output>90</output>

<gadget id="calculator">90 + 80</gadget>
<output>170</output>

<gadget id="calculator">210 - 170</gadget>
<output>40</output>

<result>40</result>