When adding two numbers, if one addend is increased by 9, to increase the sum by 18, what changes should be made to the other addend: how much.
两个数相加，如果一个加数增加9，要使和增加18，另一个加数应有什么变化：多少．
<gadget id="calculator">18 - 9</gadget>
<output>9</output>

<result>9</result>