2011 AIME I Problems/Problem 12
Problem
Six men and some number of women stand in a line in random order. Let be the probability that a group of at least four men stand together in the line, given that every man stands next to at least one other man. Find the least number of women in the line such that
does not exceed 1 percent.
Solution
Let be the number of women present, and let _ be some positive number of women between groups of men. Since the problem states that every man stands next to another man, there cannot be isolated men. Thus, there are five cases to consider, where
refers to a consecutive group of
men:
_(2)_(2)_(2)_
_(3)_(3)_
_(2)_(4)_
_(4)_(2)_
_(6)_
For the first case, we can place the three groups of men in between women. We can think of the groups of men as dividers splitting up the women. Since there are
possible places to insert the dividers, and we need to choose any three of these locations, we have
ways.
The second, third, and fourth cases are like the first, only we need to insert two dividers among the possible locations. Each gives us
ways, for a total of
.
The last case gives us ways.
Therefore, the total number of possible ways where there are no isolated men is
The total number of ways where there is a group of at least four men together is the sum of the third, fourth, and fifth case, or
Thus, we want to find the minimum possible value of where
is a positive integer such that
The numerator is equal to
For the denominator, we get
So, we get
since
must be positive. So, when multiplying both sides of the inequality by that expression, it will not change the inequality sign. After multiplying by it, we get
Thus we seek the smallest positive integer value of such that
. Since the quadratic function's discriminant, or
, is positive, the polynomial has two distinct real roots.
Also, since the polynomial has a positive leading coefficient, the graph of the polynomial is concave up, and the value of we want must be either slightly larger than the positive root (if the other, smaller root is negative) or equal to
(if the smaller root is positive). We see that
does not satisfy the inequality, so there must be a positive and negative root.
The solution to the polynomial is
We want the positive solution and find the smallest integer greater than that solution. So, we will look only at the case, not the
.
Let's look at the discriminant:
So .
Therefore, we're looking for
Since is slightly less than 1 when
is large,
will be close to
. They equal each other when
.
If we let or
, we will notice that the answer is
See also
2011 AIME I (Problems • Answer Key • Resources) | ||
Preceded by Problem 11 |
Followed by Problem 13 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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