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m1 questions [#permalink] New post 08 Nov 2009, 16:18
How many times does 7 appear in all the numbers between 1 to 1000?

I am not arriving at the answer.
Here is how I did it:
between 0 and 100, 7 appears in the following 7, 17, 27, 37, 47, 57, 67, 87, 97 (9 times+ 10 times from 70 to 79)
So a total of 19 times.
For each 100, this pattern repeats. So, I get 19*10 for everywhere between 0 and 1000
For 700 to 700, we get 100 more. So, I get 290.

What did I miss?
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Re: m1 questions [#permalink] New post 08 Nov 2009, 20:51
tusharvk wrote:
How many times does 7 appear in all the numbers between 1 to 1000?

I am not arriving at the answer.
Here is how I did it:
between 0 and 100, 7 appears in the following 7, 17, 27, 37, 47, 57, 67, 87, 97 (9 times+ 10 times from 70 to 79)
So a total of 19 times.
For each 100, this pattern repeats. So, I get 19*10 for everywhere between 0 and 1000
For 700 to 700, we get 100 more. So, I get 290.

What did I miss?


since 7s from 700 to 799 are being added , u need to ignore the 7s in 700s when multplying with 19.

so 19*9 + 100 = 271

and i think the question wants how many times 7 appears. Not how many numbers with 7 are there.

so 77 would add 2, 777 would add 3 etc.

so that makes 20 7s in a hundred.
so 20x9 = 180

now from 700-799.

u got 100(7 in the hundreds) + 10(7 in the tens) +10 (7 in the ones) = 120

so total = 180+120 = 300
_________________

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Re: m1 questions   [#permalink] 08 Nov 2009, 20:51
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