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Find the maximum value of nn such that

61×42×26×81×64×49×47×51×63×16×6761 \times 42 \times 26 \times 81 \times 64 \times 49 \times 47 \times 51 \times 63 \times 16 \times 67 is perfectly divisible by 56n56^n

Solution

✅ Correct Option: 3

Break 56 into primes:

56=23×756=2^3\times7

So we only need to know how many 2s and how many 7s the big product has.


Number2s it gives7s it gives
42=2×3×742=2\times3\times711
26=2×1326=2\times1310
64=2664=2^660
16=2416=2^440
49=7×749=7\times702
63=7×963=7\times901

The remaining numbers 6161, 8181, 4747, 5151 and 6767 contain neither 2 nor 7.

Total 2s=1+1+6+4=12\text{Total 2s}=1+1+6+4=12

Total 7s=1+2+1=4\text{Total 7s}=1+2+1=4


Every single 56 needs three 2s and one 7.

The 2s allow 123=4\dfrac{12}{3}=4 such groups.

The 7s allow 41=4\dfrac{4}{1}=4 such groups.

We must take the smaller of the two, so

n=4n=4

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