The school held a book charity sale. The income from the second grade charity sale was 420 yuan, and the sixth grade charity sale amount was 20 yuan less than that of the second grade. How much is the sixth grade charity sale amount?
学校举行图书义卖活动，二年级义卖所得420元，六年级义卖金额比二年级的2倍少20元，六年级义卖金额是多少元？
<gadget id="calculator">420 * 2</gadget>
<output>840</output>

<gadget id="calculator">840 - 20</gadget>
<output>820</output>

<result>820</result>