Maths Olympiad Prep

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Combinatorics Difficulty 4.8 AIME Prove it United States

Problem:

How many sequences of 5 positive integers (a,b,c,d,e)(a, b, c, d, e) satisfy abcdea+b+c+d+e10a b c d e \leq a+b+c+d+e \leq 10?

Solution

Solution:

We count based on how many 11's the sequence contains.

If a=b=c=d=e=1a = b = c = d = e = 1 then this gives us 11 possibility.

If a=b=c=d=1a = b = c = d = 1 and e1e \neq 1, ee can be 2,3,4,5,62, 3, 4, 5, 6. Each such sequence (1,1,1,1,e)(1,1,1,1,e) can be arranged in 55 different ways, for a total of 55=255 \cdot 5 = 25 ways in this case.

If three of the numbers are 11, the last two can be (2,2),(3,3),(2,3),(2,4)(2,2), (3,3), (2,3), (2,4), or (2,5)(2,5). Counting ordering, this gives a total of 210+320=802 \cdot 10 + 3 \cdot 20 = 80 possibilities.

If two of the numbers are 11, the other three must be equal to 22 for the product to be under 1010, and this yields 1010 more possibilities.

Thus there are 1+25+80+10=1161 + 25 + 80 + 10 = 116 such sequences.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.