The product of 6.5 and 3 plus 3 times a number, the sum is 31.2, what is this number?
6.5与3的积加上一个数的3倍，和是31.2，这个数是多少？
<gadget id="calculator">6.5 * 3</gadget>
<output>19.5</output>

<gadget id="calculator">31.2 - 19.5</gadget>
<output>11.7</output>

<gadget id="calculator">11.7 / 3</gadget>
<output>3.9</output>

<result>3.9</result>