# 1991 AHSME Problems/Problem 30

## Problem

For any set $S$, let $|S|$ denote the number of elements in $S$, and let $n(S)$ be the number of subsets of $S$, including the empty set and the set $S$ itself. If $A$, $B$, and $C$ are sets for which $n(A)+n(B)+n(C)=n(A\cup B\cup C)$ and $|A|=|B|=100$, then what is the minimum possible value of $|A\cap B\cap C|$?

$(A) 96 \ (B) 97 \ (C) 98 \ (D) 99 \ (E) 100$

## Solution 1

$n(A)=n(B)=2^{100}$, so $n(C)$ and $n(A \cup B \cup C)$ are integral powers of $2$ $\Longrightarrow$ $n(C)=2^{101}$ and $n(A \cup B \cup C)=2^{102}$. Let $A=\{s_1,s_2,s_3,...,s_{100}\}$, $B=\{s_3,s_4,s_5,...,s_{102}\}$, and $C=\{s_1,s_2,s_3,...,s_{k-2},s_{k-1},s_{k+1},s_{k+2},...,s_{100},s_{101},s_{102}\}$ where $s_k \in A \cap B$ Thus, the minimum value of $|A\cap B \cap C|$ is $\fbox{B=97}$

## Solution 2 (PIE)

As $|A|=|B|=100$, $n(A)=n(B)=2^{100}$

As $n(A)+n(B)+n(C)=n(A \cup B \cup C)$, $2^{|A|}+2^{|B|}+2^{|C|}=2^{|A \cup B \cup C|}$, $2^{100}+2^{100}+2^{|C|}=2^{|A \cup B \cup C|}$

$2^{101}+2^{|C|}=2^{|A \cup B \cup C|}$ as $|C|$ and $|A \cup B \cup C|$ are integers, $|C|=101$ and $|A \cup B \cup C| = 102$

By the Principle of Inclusion-Exclusion, $|A \cup B| = |A| + |B| - |A \cap B| = 200 - |A \cap B|$

$|A|=|B| \le |A \cup B| \le |A \cup B \cup C|$, $100 \le |A \cup B| \le 102$, $98 \le |A \cap B| \le 100$

By the Principle of Inclusion-Exclusion, $|A \cup C| = |A| + |C| - |A \cap C| = 201 - |A \cap C|$

$|C| \le |A \cup C| \le |A \cup B \cup C|$, $101 \le |A \cup C| \le 102$, $99 \le |A \cap C| \le 100$

By the Principle of Inclusion-Exclusion, $|B \cup C| = |B| + |C| - |B \cap C| = 201 - |B \cap C|$

$|C| \le |B \cup C| \le |A \cup B \cup C|$, $101 \le |B \cup C| \le 102$, $99 \le |B \cap C| \le 100$

By the Principle of Inclusion-Exclusion, $|A \cap B \cap C|=|A \cup B \cup C|- |A| - |B| - |C| + |A \cap B| + |A \cap C|+|B \cap C| = 102-100-100-101+ |A \cap B| + |A \cap C|$ $+|B \cap C|=|A \cap B| + |A \cap C|+|B \cap C| -199$

$$98 + 99 + 99 - 199 \le |A \cap B \cap C| \le 100+100+100-199$$

$$\boxed{\textbf{97}} \le |A \cap B \cap C| \le 101$$