When adding two numbers, I mistook the addend 12 as 21, and the result is 51. What should be the correct result?
两个数相加，错把加数12看成21，结果得51，正确的结果应该是多少
<gadget id="calculator">51 - 21</gadget>
<output>30</output>

<gadget id="calculator">12 + 30</gadget>
<output>42</output>

<result>42</result>