When adding two numbers, the sum is 15.9. If one addend is decreased by 6.2 and the other is increased by 2.4, what should the sum be?
两数相加，和是15.9，如果一个加数减少6.2，另一个加数增加2.4，和应变为多少．
<gadget id="calculator">15.9 - 6.2</gadget>
<output>9.7</output>

<gadget id="calculator">9.7 + 2.4</gadget>
<output>12.1</output>

<result>12.1</result>