Difference between revisions of "1991 AIME Problems/Problem 13"
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Since it doesn't matter whether the number of blue or red socks is <math>990</math>, we take the higher value for <math>r</math>, thus the maximum number of red socks is <math>r=\boxed{990}</math>. | Since it doesn't matter whether the number of blue or red socks is <math>990</math>, we take the higher value for <math>r</math>, thus the maximum number of red socks is <math>r=\boxed{990}</math>. | ||
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+ | === Solution 4 === | ||
+ | As above, let <math>r</math>, <math>b</math>, and <math>t</math> denote the number of red socks, the number of blue socks, and the total number of socks, respectively. We see that <math>\frac{r(r-1)}{t(t-1)}+\frac{b(b-1)}{t(t-1)}=\frac{1}{2}</math>, so <math>r^2+b^2-r-b=\frac{t(t-1)}{2}=r^2+b^2-t=\frac{t^2}{2}-\frac{t}{2}</math>. | ||
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+ | Seeing that we can rewrite <math>r^2+b^2</math> as <math>(r+b)^2-2rb</math>, and remembering that <math>r+b=t</math>, we have <math>\frac{t^2}{2}-\frac{t}{2}=t^2-2rb-t</math>, so <math>2rb=\frac{t^2}{2}-\frac{t}{2}</math>, which equals <math>r^2+b^2-t</math>. | ||
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+ | We now have <math>r^2+b^2-t=2rb</math>, so <math>r^2-2rb+b^2=t</math> and <math>r-b=\pm\sqrt{t}</math>. Adding this to <math>r+b=t</math>, we have <math>2r=t\pm\sqrt{t}</math>. To maximize <math>r</math>, we must use the positive square root and maximize <math>t</math>. The largest possible value of <math>t</math> is the largest perfect square less than 1991, which is <math>1936=44^2</math>. Therefore, <math>r=\frac{t+\sqrt{t}}{2}=\frac{1936+44}{2}=\boxed{990}</math>. | ||
== See also == | == See also == |
Revision as of 21:55, 23 April 2017
Contents
Problem
A drawer contains a mixture of red socks and blue socks, at most in all. It so happens that, when two socks are selected randomly without replacement, there is a probability of exactly
that both are red or both are blue. What is the largest possible number of red socks in the drawer that is consistent with this data?
Solution
Solution 1
Let and
denote the number of red and blue socks, respectively. Also, let
. The probability
that when two socks are drawn randomly, without replacement, both are red or both are blue is given by
Solving the resulting quadratic equation , for
in terms of
, one obtains that
Now, since and
are positive integers, it must be the case that
, with
. Hence,
would correspond to the general solution. For the present case
, and so one easily finds that
is the largest possible integer satisfying the problem conditions.
In summary, the solution is that the maximum number of red socks is .
Solution 2
Let and
denote the number of red and blue socks such that
. Then by complementary counting, the number of ways to get a red and a blue sock must be equal to
, so
must be a perfect square
. Clearly,
, so the larger
, the larger
:
is the largest perfect square below
, and our answer is
.
Solution 3
Let and
denote the number of red and blue socks, respectively. In addition, let
, the total number of socks in the drawer.
From the problem, it is clear that
Expanding, we get
Substituting for
and cross multiplying, we get
Combining terms, we get
To make this expression factorable, we add to both sides, resulting in
From this equation, we can test values for the expression , which is the multiplication of two consecutive integers, until we find the highest value of
or
such that
.
By testing and
, we get that
and
. Testing values one integer higher, we get that
and
. Since
is greater than
, we conclude that
is our answer.
Since it doesn't matter whether the number of blue or red socks is , we take the higher value for
, thus the maximum number of red socks is
.
Solution 4
As above, let ,
, and
denote the number of red socks, the number of blue socks, and the total number of socks, respectively. We see that
, so
.
Seeing that we can rewrite as
, and remembering that
, we have
, so
, which equals
.
We now have , so
and
. Adding this to
, we have
. To maximize
, we must use the positive square root and maximize
. The largest possible value of
is the largest perfect square less than 1991, which is
. Therefore,
.
See also
1991 AIME (Problems • Answer Key • Resources) | ||
Preceded by Problem 12 |
Followed by Problem 14 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.