Difference between revisions of "2011 AMC 10A Problems/Problem 23"

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Problem

Seven students count from 1 to 1000 as follows:

•Alice says all the numbers, except she skips the middle number in each consecutive group of three numbers. That is, Alice says 1, 3, 4, 6, 7, 9, . . ., 997, 999, 1000.

•Barbara says all of the numbers that Alice doesn't say, except she also skips the middle number in each consecutive group of three numbers.

•Candice says all of the numbers that neither Alice nor Barbara says, except she also skips the middle number in each consecutive group of three numbers.

•Debbie, Eliza, and Fatima say all of the numbers that none of the students with the first names beginning before theirs in the alphabet say, except each also skips the middle number in each of her consecutive groups of three numbers.

•Finally, George says the only number that no one else says.

What number does George say?

$\textbf{(A)}\ 37\qquad\textbf{(B)}\ 242\qquad\textbf{(C)}\ 365\qquad\textbf{(D)}\ 728\qquad\textbf{(E)}\ 998$

Solution

First look at the numbers Alice says. $1, 3, 4, 6, 7, 9 \cdots$ skipping every number that is congruent to $2 \pmod 3$ . Thus, Barbara says those numbers EXCEPT every second - being $2 + 3^1 \equiv 5 \pmod{3^2=9}$ . So Barbara skips every number congruent to $5 \pmod 9$ . We continue and see:

Alice skips $2 \pmod 3$ , Barbara skips $5 \pmod 9$ , Candice skips $14 \pmod {27}$ , Debbie skips $41 \pmod {81}$ , Eliza skips $122 \pmod {243}$ , and Fatima skips $365 \pmod {729}$ .

Since the only number congruent to $365 \pmod {729}$ and less than $1,000$ is $365$ , the correct answer is $\boxed{365\ \mathbf{(C)}}$ .

Solution 2

After Alice says all her numbers, the numbers not mentioned yet are $\text{Alice: } 2,5,8,11,14,17,\cdots,998.$ After Barbara says all her numbers, the numbers that haven't been said yet are $\text{Barbara: } 5,14,23,32,41,50,\cdots,995.$ After Candice, the list is $\text{Candice: } 14,41,68,\cdots,986.$ Notice how each list is an arithmetic sequence where the common ratio is thrice the common ratio of the previous list and the first term is the second term of the previous list. Now that the pattern is clear, we construct the rest of the lists: $\text{Debbie: } 41,122,203,\cdots,959$ $\text{Eliza: } 122,365,608,878$ $\text{Fatima: } \boxed{\textbf{(C) } 365}$

Solution 3

Notice that Alice has skipped the numbers $3n-1$ for $n=1,2,3,...,333$ . Namely, $3\cdot1-1,3\cdot2-1,3\cdot3-1,...,3\cdot333-1$ Thus the numbers that Barbara skips are $3\cdot2-1,3\cdot5-1,3\cdot8-1,...$ or in a more general expression, $3(3n-1)-1$ for $n=1,2,3,...$ . Namely, $3\cdot(3\cdot1-1)-1,3\cdot(3\cdot2-1)-1,3\cdot(3\cdot3-1)-1,...$ Repeating the pattern until George, we have the first number he says, $3(3(3(3(3(3\cdot1-1)-1)-1)-1)-1)-1=365 \boxed{\mathbf{C}}$ In addition, note that the second number George says exceeds 1000.

~ Nafer

2011 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 22	Followed by Problem 24
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