2011 AIME II Problems/Problem 6

Problem 6

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

Solution 1

Rearranging the inequality we get $d-c > b-a$ . Let $e = 11$ , then $(a, b-a, c-b, d-c, e-d)$ is a partition of 11 into 5 positive integers or equivalently: $(a-1, b-a-1, c-b-1, d-c-1, e-d-1)$ is a partition of 6 into 5 non-negative integer parts. Via a standard stars and bars argument, the number of ways to partition 6 into 5 non-negative parts is $\binom{6+4}4 = \binom{10}4 = 210$ . The interesting quadruples correspond to partitions where the second number is less than the fourth. By symmetry, there are as many partitions where the fourth is less than the second. So, if $N$ is the number of partitions where the second element is equal to the fourth, our answer is $(210-N)/2$ .

We find $N$ as a sum of 4 cases:

two parts equal to zero, $\binom82 = 28$ ways,
two parts equal to one, $\binom62 = 15$ ways,
two parts equal to two, $\binom42 = 6$ ways,
two parts equal to three, $\binom22 = 1$ way.

Therefore, $N = 28 + 15 + 6 + 1 = 50$ and our answer is $(210 - 50)/2 = \fbox{080}$

Solution 2

Let us consider our quadruple (a,b,c,d) as the following image xaxbcxxdxx. The location of the letter a,b,c,d represents its value and x is a place holder. Clearly the quadruple is interesting if there are more place holders between c and d than there are between a and b. 0 holders between a and b means we consider a and b as one unit ab and c as cx yielding $\binom83 = 56$ ways, 1 holder between a and b means we consider a and b as one unit axb and c as cxx yielding $\binom 63 = 20$ ways, 2 holders between a and b means we consider a and b as one unit axxb and c as cxxx yielding $\binom43 = 4$ ways and there cannot be 3 holders between a and b so our total is 56+20+4= $\fbox{080}$ .

Solution 3 (Slightly bashy)

We first start out when the value of $a=1$ .

Doing casework, we discover that $d=5,6,7,8,9,10$ . We quickly find a pattern.

Now, doing this for the rest of the values of $a$ and $d$ , we see that the answer is simply:

$(1)+(2)+(1+3)+(2+4)+(1+3+5)+(2+4+6)+(1)+(2)+(1+3)+(2+4)$ $+(1+3+5)+(1)+(2)+(1+3)+(2+4)+(1)+(2)+(1+3)+(1)+(2)+(1)=\boxed{080}$

Solution 4 (quick)

Notice that if $a+d>b+c$ , then $(11-a)+(11-d)<(11-b)+(11-c)$ , so there is a 1-to-1 correspondence between the number of ordered quadruples with $a+d>b+c$ and the number of ordered quadruples with $a+d<b+c$ .

Quick counting gives that the number of ordered quadruples with $a+d=b+c$ is 50. To count this, consider our numbers $1, 2, 3, 4, 5, 6, 7, 8, 9 10$ . Notice that if, for example, $a+d=b+c=8$ , that the average of $a,d$ and $b,c$ must both be $4$ . In this way, there is a symmetry for this case, centered at $4$ . If instead, say, $a+d=b+c=7$ , an odd number, then there is symmetry with $(a,d);(b,c)$ about $3.5$ . Further, the number of cases for each of these centers of symmetry correspond to a triangular number. Eg centered at $2.5,3,8,8.5$ , there is $1$ case for each and so on, until centered at $5.5$ , there are $10$ possible cases. Adding these all, we have $2(1+3+6)+10=50$ .

Thus the answer is $\frac{\binom{10}{4}-50}{2} = \boxed{080}.$

Solution 5

Think about a,b,c,and d as distinct objects, that we must place in 4 of 10 spaces. However, in only 1 of 24 of these combinations, will the placement of these objects satisfy the condition in the problem. So we know the total number of ordered quadruples is $(10*9*8*7/24)=210$

Next, intuitively, the number of quadruples where $a+d>b+c$ is equal to the number of quadruples where $a+d<b+c$ . So we need to find the number of quadruples where the two quantities are equal. To do this, all we have to do is consider the cases when $a-d$ ranges from 3 to 9. It would seem natural that a range of 3 would produce 1 option, and a range of 4 would produce 2 options. However, since b and c cannot be equal, a range of 3 or 4 produces 1 option each, a range of 5 or 6 produces 2 options each, a range of 7 or 8 produces 3 options each, and a range of 9 will produce 4 options. In addition, a range of n has 10-n options for combinations of a and d. Multiplying the number of combinations of a and d by the corresponding number of options for b and c gives us 50 total quadruplets where $a+d=b+c$ .

So the answer will be $\frac{210-50}{2} = \boxed{080}.$

2011 AIME II (Problems • Answer Key • Resources)
Preceded by Problem 5	Followed by Problem 7
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