Divide the seven numbers 1, 2, 3, 4, 5, 6, and 7 into two groups, form a three-digit number and a four-digit number, and make the product of these two numbers the largest, then what is the three-digit number ．
将1，2，3，4，5，6，7这七个数分成两组，组成一个三位数和一个四位数，并使这两个数的乘积最大，那么这个三位数是多少．
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