The difference between 320 and 280 is divided by their sum, what is the comprehensive formula.
320与280的差除他们的和，列综合算式是多少．
<gadget id="calculator">320 + 280</gadget>
<output>600</output>

<gadget id="calculator">320 - 280</gadget>
<output>40</output>

<gadget id="calculator">600 / 40</gadget>
<output>15</output>

<result>15</result>