Maths Olympiad Prep

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Combinatorics Difficulty 5.6 AIME, harder Prove it Japan

How many quadruples (a,b,c,d)(a, b, c, d) of positive integers lying in between 11 and 99 (11 and 99 inclusive) are there which satisfy 0<ba<cb<dc0 < b - a < c - b < d - c?

Solution

Let x=bax = b - a, y=cby = c - b, z=dcz = d - c. Then, x,y,zx, y, z are integers satisfying 0<x<y<z0 < x < y < z. Furthermore, since x+y+z=da91=8x + y + z = d - a \le 9 - 1 = 8, we see that the triple (x,y,z)(x, y, z) must be one of the following 44 triples:
(1,2,3), (1,2,4), (1,2,5), (1,3,4). (1, 2, 3),\ (1, 2, 4),\ (1, 2, 5),\ (1, 3, 4).
When (x,y,z)=(1,2,3)(x, y, z) = (1, 2, 3), we get (a,b,c,d)=(a,a+1,a+3,a+6)(a, b, c, d) = (a, a+1, a+3, a+6). So, corresponding to the cases a=1,2,3a = 1, 2, 3, we get 33 quadruples (a,b,c,d)(a, b, c, d) satisfying the required condition. Similarly, when (x,y,z)=(1,2,4)(x, y, z) = (1, 2, 4), we get 22 quadruples (a,b,c,d)(a, b, c, d) satisfying the requirement corresponding to the cases a=1,2a = 1, 2. We also find if (x,y,z)=(1,2,5)(x, y, z) = (1, 2, 5) or if (x,y,z)=(1,3,4)(x, y, z) = (1, 3, 4) only 11 quadruple (a,b,c,d)(a, b, c, d) corresponding to a=1a = 1 will satisfy the requirement. Therefore, there are only 3+2+1+1=73 + 2 + 1 + 1 = 7 quadruples (a,b,c,d)(a, b, c, d) satisfying the requirement.

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