(□-21)/13+6=72, how much should be filled in □
(□-21)/13+6=72，□里应填多少
<gadget id="calculator">72 - 6</gadget>
<output>66</output>

<gadget id="calculator">66 * 13</gadget>
<output>858</output>

<gadget id="calculator">858 + 21</gadget>
<output>879</output>

<result>879</result>