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USAMO
2011
Problem - 3438
Let
A
be a set with
|
A
|
=
225
, meaning that
A
has 225 elements. Suppose further that there are eleven subsets
A
1
,
…
,
A
11
of
A
such that
|
A
i
|
=
45
for
1
≤
i
≤
11
and
|
A
i
∩
A
j
|
=
9
for $1\leq i
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