2011 AIME II Problem 6

Below is the professionally curated solution for Problem 6 of the 2011 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2011 AIME II solutions, or check the answer key.

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Concepts:combinationsbijectionsymmetry

Difficulty rating: 2390

6.

Define an ordered quadruple of integers (a,b,c,d)(a, b, c, d) to be interesting if 1a<b<c<d101 \le a \lt b \lt c \lt d \le 10 and a+d>b+c.a + d \gt b + c. How many interesting ordered quadruples are there?

Solution:

The condition a+d>b+ca + d \gt b + c is equivalent to dc>ba.d - c \gt b - a. There are (104)=210\binom{10}{4} = 210 quadruples in all, and the involution (a,b,c,d)(11d,11c,11b,11a)(a, b, c, d) \mapsto (11 - d,\, 11 - c,\, 11 - b,\, 11 - a) exchanges the outer gaps bab - a and dc.d - c. So the quadruples with dc>bad - c \gt b - a and those with dc<bad - c \lt b - a are equinumerous, and the answer is 210T2,\frac{210 - T}{2}, where TT counts quadruples with dc=ba.d - c = b - a.

If ba=dc=kb - a = d - c = k and cb=j,c - b = j, the quadruple is determined by (a,j,k)(a, j, k) with a,j,k1a, j, k \ge 1 and a+2k+j10.a + 2k + j \le 10. For k=1,2,3,4k = 1, 2, 3, 4 the pairs (a,j)(a, j) with a+j8,6,4,2a + j \le 8, 6, 4, 2 number 28,28, 15,15, 6,6, 1,1, so T=50.T = 50.

Therefore the number of interesting quadruples is 210502=80.\frac{210 - 50}{2} = 80.

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