When a=4, b=7, c=5, what is the value of bc-ac.
当a=4，b=7，c=5时，bc-ac的值是多少．
<gadget id="calculator">7 * 5</gadget>
<output>35</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">35 - 20</gadget>
<output>15</output>

<result>15</result>