When two numbers are subtracted, the minuend is increased by 20, and the subtrahend is decreased by 10. What is the change in the difference: how much.
两个数相减，被减数增加20，减数减少10，差有什么变化：多少．
<gadget id="calculator">50 - 30</gadget>
<output>20</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

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

<gadget id="calculator">50 + (-20)</gadget>
<output>30</output>

<result>30</result>