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integers

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Source: — Problem Solving |

by Brent@GMATPrepNow » Sat Sep 01, 2018 6:13 am
vaibhav101 wrote:how many numbers of times will the digit 7 be written when listing the integers from 1 to 1000?

A 271
B 300
C 252
D 304
E 204
Here's one way to look at it.
Write all of the numbers as 3-digit numbers.
That is, 000, 001, 002, 003, .... 998, 999

NOTE: Yes, I have started at 000 and ended at 999, even though though the question asks us to look at the numbers from 1 to 1000. HOWEVER, notice that 000 and 1000 do not have any 7's so the outcome will be the same.

First, there are 1000 integers from 000 to 999
There are 3 digits in each integer.
So, there is a TOTAL of 3000 individual digit. (since 1000 x 3 = 3000)

Each of the 10 digits is equally represented, so the 7 will account for 1/10 of all digits.

1/10 of 3000 = 300

So, there are 300 0's, 300 1's, 300 2's, 300 3's, . . ., and 300 9's in the integers from 000 to 999

Answer: B

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by GMATGuruNY » Sat Sep 01, 2018 6:25 am
vaibhav101 wrote:how many numbers of times will the digit 7 be written when listing the integers from 1 to 1000?

A 271
B 300
C 252
D 304
E 204
If we use 0 as a placeholder -- so that 007 represents 7, 072 represents 72, etc. -- then we need to count the number of times that 7 will appear among the 3-digit integers from 000 to 999, inclusive.

Total number of 3-digit integers from 000 to 999, inclusive = biggest - smallest + 1 = 999-000+1 = 1000.

Each of these 1000 integers includes 3 digits.
Thus, the total number of digit appearances = 3*1000 = 3000.

Among these 3000 digit appearances, each of the 10 digits will appear the same number of times.
Thus, the number of times that 7 will appear = 3000/10 = 300.

The correct answer is B.
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by Scott@TargetTestPrep » Tue Sep 11, 2018 4:19 pm
vaibhav101 wrote:how many numbers of times will the digit 7 be written when listing the integers from 1 to 1000?

A 271
B 300
C 252
D 304
E 204
The digit 7 as the hundreds digit appears 100 times (700 to 799). As the tens digit, it appears 100 times also (ten times each in the 70s, 170s, 270s, ..., 970s). As the units digit, it appears 100 times also (7, 17, 27, ..., 997). Therefore, the digit 7 has appeared a total of 300 times.

Answer: B

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