2017 AMC 12A Problems/Problem 5

Problem

At a gathering of $30$ people, there are $20$ people who all know each other and $10$ people who know no one. People who know each other hug, and people who do not know each other shake hands. How many handshakes occur?

$\textbf{(A)}\ 240\qquad\textbf{(B)}\ 245\qquad\textbf{(C)}\ 290\qquad\textbf{(D)}\ 480\qquad\textbf{(E)}\ 490$

Solution - Basic

All of the handshakes will involve at least one person from the $10$ who know no one. Label these ten people $A$ , $B$ , $C$ , $D$ , $E$ , $F$ , $G$ , $H$ , $I$ , $J$ .

Person $A$ from the group of 10 will initiate a handshake with everyone else ( $29$ people). Person $B$ initiates $28$ handshakes plus the one already counted from person $A$ . Person $C$ initiates $27$ new handshakes plus the two we already counted. This continues until person $J$ initiates $20$ handshakes plus the nine we already counted from $A$ ... $I$ .

$29+28+27+26+25+24+23+22+21+20 = \boxed{(B)=\ 245}$

Solution

Let the group of people who all know each other be $A$ , and let the group of people who know no one be $B$ . Handshakes occur between each pair $(a,b)$ such that $a\in A$ and $b\in B$ , and between each pair of members in $B$ . Thus, the answer is

$|A||B|+{|B|\choose 2} = 20\cdot 10+{10\choose 2} = 200+45 = \boxed{(B)=\ 245}$

Solution - Complementary Counting

The number of handshakes will be equivalent to the difference between the number of total interactions and the number of hugs, which are ${30\choose 2}$ and ${20\choose 2}$ , respectively. Thus, the total amount of handshakes is ${30\choose 2} - {20\choose 2} = 435 - 190= \boxed{(B)=\ 245}$

Solution

Each person who does not know anybody will shake hands with all 20 of the people who know each other. This means there will be at least $20 * 10 = 200$ handshakes. In addition, the entire group of people who don't know anyone will shake hands with each other, giving another $9+8+7+6+5+4+3+2+1 = 45$ handshakes. Therefore, there is a total of $200+45 = 245 handshakes \boxed{(B)}$.

2017 AMC 10A (Problems • Answer Key • Resources)
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2017 AMC 12A (Problems • Answer Key • Resources)
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All AMC 12 Problems and Solutions

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2017 AMC 12A Problems/Problem 5

Contents

Problem

Solution - Basic

Solution

Solution - Complementary Counting

Solution

See Also