Answer :
The largest number is 952. I'll tell you why...
7*8=56
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56*18=7*8*18=7*8*3*6=1008 (4 digits)
56*17=7*8*17=952 (3 digits)
7*8=56
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56*18=7*8*18=7*8*3*6=1008 (4 digits)
56*17=7*8*17=952 (3 digits)