Find the law and fill in the numbers: 0, 3, 8, 15, how much.
找规律填数：0、3、8、15、多少．
<gadget id="calculator">7 + 2</gadget>
<output>9</output>

<gadget id="calculator">15 + 9</gadget>
<output>24</output>

<result>24</result>