The sum of three numbers is 31.5, the first number is twice the second number, the second number is twice the third number, how much is the first number greater than the second number.
三个数的和是31.5，第一个数是第二数的2倍，第二个数是第三个数的2倍，第一个数比第二个数多多少．
<gadget id="calculator">4 + 2 + 1</gadget>
<output>7</output>

<gadget id="calculator">31.5 / 7</gadget>
<output>4.5</output>

<gadget id="calculator">4 * 4.5</gadget>
<output>18</output>

<gadget id="calculator">2 * 4.5</gadget>
<output>9</output>

<gadget id="calculator">18 - 9</gadget>
<output>9</output>

<result>9</result>