2011 AIME II Problem 6

Attempt Problem 6 of the 2011 AIME II below, then check your answer against the professionally curated solution 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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6.

Define an ordered quadruple of integers (a,b,c,d)(a, b, c, d) to be interesting if 1≤a<b<c<d≤101 \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?

Answer: 80
Concepts:combinationsbijectionsymmetry
Difficulty rating: 2390
Small Hint:

The condition a+d>b+ca + d \gt b + c is the same as d−c>b−ad - c \gt b - a

Big Hint:

The map (a,b,c,d)(a, b, c, d) ↦(11−d, 11−c, 11−b, 11−a)\small \mapsto (11 - d,\, 11 - c,\, 11 - b,\, 11 - a) swaps d−cd - c with b−a;b - a; count the quadruples with d−c=b−ad - c = b - a and use symmetry

Solution:

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

If b−a=d−c=kb - a = d - c = k and c−b=j,c - b = j, the quadruple is determined by (a,j,k)(a, j, k) with a,j,k≥1a, j, k \ge 1 and a+2k+j≤10.a + 2k + j \le 10. For k=1,k = 1, 2,2, 3,3, and 4,4, respectively, the bounds on a+ja + j are 8,8, 6,6, 4,4, and 2;2; the corresponding counts are 28,28, 15,15, 6,6, and 1,1, so T=50.T = 50.

Therefore the number of interesting quadruples is 210−502=80.\frac{210 - 50}{2} = 80.

Problem 5#5
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