How much greater is the sum of 12.9 and 2.1 than their difference
12.9与2.1的和比它们的差大多少
<gadget id="calculator">12.9 + 2.1</gadget>
<output>15</output>

<gadget id="calculator">12.9 - 2.1</gadget>
<output>10.8</output>

<gadget id="calculator">15 - 10.8</gadget>
<output>4.2</output>

<result>4.2</result>