When a number is divided by 31, the quotient is 20 and the remainder is 18. What is the number?
一个数除以31，商是20，余数是18，这个数是多少．
<gadget id="calculator">31 * 20</gadget>
<output>620</output>

<gadget id="calculator">620 + 18</gadget>
<output>638</output>

<result>638</result>