Find the law and fill in the numbers: 1, 3, 7, 15, how much.
找规律填数：1、3、7、15、多少．
<gadget id="calculator">8 * 2</gadget>
<output>16</output>

<gadget id="calculator">15 + 16</gadget>
<output>31</output>

<result>31</result>