Three times the sum of a number plus 0.5 is equal to 21. Find this number.
一个数加0.5的和的3倍等于21，求这个数．
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">7 - 0.5</gadget>
<output>6.5</output>

<result>6.5</result>