□/21=18…△, when △ is the largest, what is □ at this time?
□/21=18…△中，当△最大时，这时□是多少．
<gadget id="calculator">18 * 21</gadget>
<output>378</output>

<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">378 + 20</gadget>
<output>398</output>

<result>398</result>