There are 8 green balloons and 7 blue balloons. The number of blue balloons remains the same. If you add more green balloons, the number of green balloons will be three times the number of blue balloons.
绿气球有8个，蓝气球有7个，蓝气球个数不变，再添多少个绿气球，绿气球的个数就是蓝气球的3倍．
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 8</gadget>
<output>13</output>

<result>13</result>