It is known that a=3, b=4, c=5, bc-ac= how many
已知a=3，b=4，c=5，bc-ac=多少
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<result>5</result>