If ab+cd=79 (a, b, c, d are not equal to each other), then what is the maximum value of a*b*c*d.
如果ab+cd=79(a，b，c，d互不相等)，那么，a*b*c*d的最大值是多少．
<gadget id="calculator">3 * 4 * 4 * 5</gadget>
<output>240</output>

<result>240</result>