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2003 AIME I Problems/Problem 13

Revision as of 09:42, 29 September 2018 by Novus677 (talk | contribs) (→‎Solution 2)

Problem

Let $N$ be the number of positive integers that are less than or equal to $2003$ and whose base-$2$ representation has more $1$'s than $0$'s. Find the remainder when $N$ is divided by $1000$.

Solution 1

In base-$2$ representation, all positive numbers have a leftmost digit of $1$. Thus there are ${n \choose k}$ numbers that have $n+1$ digits in base $2$ notation, with $k+1$ of the digits being $1$'s.

In order for there to be more $1$'s than $0$'s, we must have $k+1 > \frac{d+1}{2} \Longrightarrow k > \frac{d-1}{2} \Longrightarrow k \ge \frac{d}{2}$. Therefore, the number of such numbers corresponds to the sum of all numbers on or to the right of the vertical line of symmetry in Pascal's Triangle, from rows $0$ to $10$ (as $2003 < 2^{11}-1$). Since the sum of the elements of the $r$th row is $2^r$, it follows that the sum of all elements in rows $0$ through $10$ is $2^0 + 2^1 + \cdots + 2^{10} = 2^{11}-1 = 2047$. The center elements are in the form ${2i \choose i}$, so the sum of these elements is $\sum_{i=0}^{5} {2i \choose i} = 1 + 2 +6 + 20 + 70 + 252 = 351$.

The sum of the elements on or to the right of the line of symmetry is thus $\frac{2047 + 351}{2} = 1199$. However, we also counted the $44$ numbers from $2004$ to $2^{11}-1 = 2047$. Indeed, all of these numbers have at least $6$ $1$'s in their base-$2$ representation, as all of them are greater than $1984 = 11111000000_2$, which has $5$ $1$'s. Therefore, our answer is $1199 - 44 = 1155$, and the remainder is $\boxed{155}$.

Solution 2

We seek the number of allowed numbers which have $k$ 1's, not including the leading 1, for $k=0, 1, 2, \ldots , 10$.

For $k=0,\ldots , 4$, this number is

$\binom{k}{k}+\binom{k+1}{k}+\cdots+\binom{2k}{k}$.

By the Hockey Stick Identity, this is equal to $\binom{2k+1}{k+1}$. So we get

$\binom{1}{1}+\binom{3}{2}+\binom{5}{3}+\binom{7}{4}+\binom{9}{5}=175$.

For $k=5,\ldots , 10$, we end on $\binom{10}{k}$ - we don't want to consider numbers with more than 11 digits. So for each $k$ we get

$\binom{k}{k}+\binom{k+1}{k}+\ldots+\binom{10}{k}=\binom{11}{k+1}$

again by the Hockey Stick Identity. So we get

$\binom{11}{6}+\binom{11}{7}+\binom{11}{8}+\binom{11}{9}+\binom{11}{10}+\binom{11}{11}=\frac{2^{11}}{2}=2^{10}=1024$.

The total is $1024+175=1199$. Subtracting out the $44$ numbers between $2003$ and $2048$ gives $1155$. Thus the answer is $155$.

See also

2003 AIME I (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

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