Maths Olympiad Prep

Library / /38 of 377

Combinatorics Difficulty 4.4 AIME Find the answer United States

Problem:
Find the number of ordered triples (a,b,c)(a, b, c) of pairwise distinct integers such that 31a,b,c31-31 \leq a, b, c \leq 31 and a+b+c>0a+b+c>0.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
Answer: 117690
We will find the number of such triples with a<b<ca<b<c. The answer to the original problem will then be six times what we will get. By symmetry, the number of triples (a,b,c)(a, b, c) with a+b+c>0a+b+c>0 is equal to the number of those with a+b+c<0a+b+c<0. Our main step is thus to find the number of triples with sum 00.

If b=0b=0, then a=ca=-c, and there are 3131 such triples. We will count the number of such triples with b>0b>0 since the number of those with b<0b<0 will be equal by symmetry.

For all positive nn such that 1n151 \leq n \leq 15, if a=2na=-2n, there are n1n-1 pairs (b,c)(b, c) such that a+b+c=0a+b+c=0 and b>0b>0, and for all positive nn such that 1n161 \leq n \leq 16, if a=2n+1a=-2n+1, there are also n1n-1 such pairs (b,c)(b, c). In total, we have 1+1+2+2+3+3++14+14+15=2251+1+2+2+3+3+\ldots+14+14+15=225 triples in the case b>0b>0 (and hence likewise for b<0b<0.)

In total, there are 31+225+225=48131+225+225=481 triples such that a<b<ca<b<c and a+b+c=0a+b+c=0. Since there are (633)=39711\binom{63}{3}=39711 triples (a,b,c)(a, b, c) such that 31a<b<c31-31 \leq a<b<c \leq 31, the number of triples with the additional restriction that a+b+c>0a+b+c>0 is 397114812=19615\frac{39711-481}{2}=19615. So the answer to the original problem is 19615×6=11769019615 \times 6=117690.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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