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Let's break down the pattern into sections:
1, 3, 5, 7, 9, 11, 13, ..., 99, 101, 103, ..., 999

The first section consists of the single-digit odd integers: 1, 3, 5, 7, 9. Each digit appears once, so the total number of digits in this section is 5.

The second section consists of the two-digit odd integers: 11, 13, ..., 99. There are 45 two-digit odd integers, and each number has two digits, so the total number of digits in this section is 45 * 2 = 90.

Therefore, the first 95 digits of the string come from the first two sections: 5 digits from the single-digit section and 90 digits from the two-digit section.

13579.....99 (total 95 digits) -----

13579.....99 next section takes 3 places so (101...109) adds up 15 places


13579.....99101103105....109

9 is the 110th digit in the sequence
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single digit odd will be 5
2 digit odd will be 11 to 99 ; 2*5*9 ; 90
95 digit string from 1 to 99
110-95 ; 15
101 103 105 107 109 ; 9 will be the 105th digit from left
option E

rakman123
The string of digits 135791113...999 is formed by merging together the decimal representations of the odd integers from 1 through 999. Counting from left, what is the 110th digit of this string of digits?

A) 0
B) 3
C) 5
D) 7
E) 9
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rakman123
The string of digits 135791113...999 is formed by merging together the decimal representations of the odd integers from 1 through 999. Counting from left, what is the 110th digit of this string of digits?

A) 0
B) 3
C) 5
D) 7
E) 9

From 1 to 99 there are 50 odd integers, consisting of 5 single-digit and 45 two-digit odd integers.

So far, that’s a total of 5 + 45 × 2 =95 digits.

We still need 110 – 95 = 15 digits, which can be exactly provided by the first five (=15/3) three-digit odd integers.

The fifth three-digit odd integer is 109, whose last digit is 9.

Therefore, the 110th digit in question is 9.

Answer: E
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Can anyone explain what the meaning of "decimal representations" in this question? Seems these numbers are represented as integers being combined to form a continuous string.
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rakman123
The string of digits 135791113...999 is formed by merging together the decimal representations of the odd integers from 1 through 999. Counting from left, what is the 110th digit of this string of digits?

A) 0
B) 3
C) 5
D) 7
E) 9

An alternate approach is to write out the digits in ROWS OF 10 and look for a pattern:

1 3 5 7 9 1 1 1 3 1
5 1 7 1 9 2 1 2 3 2
5 2 7 2 9 3 1 3 3 3

As illustrated by the digits in blue, the row endings exhibit the following pattern:
1st row --> 31 (where 1 is the tens digit for 15)
2nd row --> 32 (where 2 is the tens digit for 25)
3rd row --> 33 (where the second 3 is the tens digit for 35)
Implication:
9th row --> 39 (where 9 is the tens digit for 95)

The 9th 10-digit row brings the total number of digits to 90.
New two rows, beginning with the units digit for 95:
5 9 7 9 9 1 0 1 1 0
3 1 0 5 1 0 7 1 0 9

The digit in green is the 110th in the string.

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wtstone
The string of digits 135791113...999 is formed by merging together the decimal representations of the odd integers from 1 through 999. Counting from left, what is the 110th digit of this string of digits?

A) 0
B) 3
C) 5
D) 7
E) 9


Can anyone explain what the meaning of "decimal representations" in this question? Seems these numbers are represented as integers being combined to form a continuous string.

Base-10 notation, also known as decimal notation, is a method of representing numbers using ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. This is the conventional system for denoting numbers, in contrast to systems like the binary numeral system, which is a base-2 number system.

So, the odd integers from 1 through 999, written as the string of digits would simply be odd numbers written one after another: 135791113...999
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I attached my work to this problem. Hope it is helpful!
Attachments

String Digits Problem GMAT.pdf [364.14 KiB]
Downloaded 151 times

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1. One digit numbers length. 1-9=5

2. Two digit numbers length
99-11/2+1= 45 numbers
45*2=90

3. Total length so far for 1-99 digits= 5+90=95
110-95=15 left

If we count from 99-- 101 103 105 107 109
9 gets us to digit 110

Answer is 9­
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Everybody here is writing explanations that require formulas and extra things to memorize.
just think about it logically

1,3,5,7,9 is 5 numbers
11,13,15,17,19, is 10 number

99 will bring you to 90 numbers. 90+5=95. 99 is the 95th number

101,103,105,107,109 = 15 numbers. 95+15=110. the final number is 9.
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ITs a little unclear. I solved this by counting 1-9 as 5 contributions, 11-19 as 10 contributions bcas its a fused string, so "11" is 2 numbers IMO. I am getting the interpretation wrong at times sigh :/
rakman123
The string of digits 135791113...999 is formed by merging together the decimal representations of the odd integers from 1 through 999. Counting from left, what is the 110th digit of this string of digits?

A) 0
B) 3
C) 5
D) 7
E) 9
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The number of single digit number use = 5 {1, 3, 5, 7, 9}

Total Digit count = 5

Number of ODD two-digit numbers from 1 to 100 = 45

Digit count = 45*2 = 90

Total Digit count so far from 1 to 99 = 5+90 = 95

i.e. Now we just have to reach 15th digit from here to get 115th digit

All numbers from hereon are three-digit numbers (till we get answer)

so 5th 3-digit number's unit digit is our answer

{101, 103, 105, 107, 109}

115th Digit = 9

Answer: Option E


rakman123
The string of digits 135791113...999 is formed by merging together the decimal representations of the odd integers from 1 through 999. Counting from left, what is the 110th digit of this string of digits?

A) 0
B) 3
C) 5
D) 7
E) 9
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Let me know if my method is just a coincidence please.

We notice the difference is 2. Also we notice the following pattern:
3rd digit of the string is 2*3=6 - 1 = 5.
5th digit of the sting is 2*5 = 10 - 1 = 9.

Thus,
110th is 2*110 = 220 - 1 =219 so answer choice E.

gmatophobia
rakman123
The string of digits 135791113...999 is formed by merging together the decimal representations of the odd integers from 1 through 999. Counting from left, what is the 110th digit of this string of digits?

A) 0
B) 3
C) 5
D) 7
E) 9

A good one!

Let's visualize this

\(\quad1 \quad3 \quad5 \quad7 \quad9 \quad11 \quad13 \quad....\quad 997 \quad 999\)

This is an arithmetic sequence with a common difference = 2

We want to find the 110th digit of this string (counting from the left)

Length of the string contributed by the 1 digit number = 5 (i.e. 13579)

Each of the two-digit numbers will contribute a length of two to the string

Total length contributed by the two-digit numbers = Number of two-digit numbers * 2

Number of two-digit numbers =\(\frac{ (99 - 11) }{ 2 }+ 1 = 45 \)

length contributed by the two-digit numbers = 45 * 2 = 90

Total Length obtained so far = 90 + 5 = 95

Remaining length = 110 - 95 = 15

Each three-digit number will contribute a length of 3 to the string. Hence, we need \(\frac{15}{3} = 5\) three digit numbers.

101 103 105 107 109

110th digit = 9

Option E
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