In a subtraction formula, the minuend is reduced by 30 and the subtrahend is increased by 30, the difference is the same.
在一个减法算式中，被减数减少30，减数增加30，差就多少．
<gadget id="calculator">70 - 40</gadget>
<output>30</output>

<gadget id="calculator">100 - 10 - 30</gadget>
<output>60</output>

<result>60</result>