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9 years, 3 months ago
9 years, 3 months ago
The number of 7s that 42! has is [$$\frac{42}{7}$$] = 6
The number of 7s that 56! has is [$$\frac{56}{7}$$] + [$$\frac{56}{7^2}$$] = 9
So, all the values from 42! to 55! are multiples of $$7^6$$