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Combinatorics Difficulty 6.0 AIME, harder Prove it Mongolia

Let aa, bb, cc, dd, ee be not necessarily distinct divisors of 210210. Find all 55-permutations (a,b,c,d,e)(a, b, c, d, e) which satisfy the condition abcde>44100abcde > 44100.

Solution

Let aa, bb, cc, dd, ee, ff be not necessarily distinct divisors of 210210. First we will find number of 66-permutations (a,b,c,d,e,f)(a, b, c, d, e, f) that satisfy the condition abcdef>2103abcdef > 210^3.
Since abcdef>2103210a210b210c210d210e210f<2103. \text{Since } abcdef > 210^3 \Leftrightarrow \frac{210}{a} \cdot \frac{210}{b} \cdot \frac{210}{c} \cdot \frac{210}{d} \cdot \frac{210}{e} \cdot \frac{210}{f} < 210^3.
Number of 66-permutations which satisfy the condition abcdef>2103abcdef > 210^3 equals to number of 66-permutations which satisfy the condition abcdef<2103abcdef < 210^3. Number of 66-permutations with abcdef=23335373abcdef = 2^3 \cdot 3^3 \cdot 5^3 \cdot 7^3 equals to ((63))4(\binom{6}{3})^4. Therefore number of 66-permutations (a,b,c,d,e,f)(a, b, c, d, e, f) which satisfy the condition abcdef>2103abcdef > 210^3 equals to (16)6((63))42\frac{(16)^6 - (\binom{6}{3})^4}{2}.

Now we apply this result to the given problem. Setting f=210f = 210 we get abcde>2102abcde > 210^2 and thus desired number is 16(16)6((63))42\frac{1}{6} \cdot \frac{(16)^6 - (\binom{6}{3})^4}{2}.

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