It is known that a△b means twice a minus half of b, for example: 3△4=3*2-4/2=4. Please find out how much 15△20= according to the above rules.
已知a△b表示a的2倍减去b的一半，例如：3△4=3*2-4/2=4．请你根据以上的规定，求15△20=多少．
<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<result>20</result>