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yes I was able to do it within a minute but that might be because I got the underlying idea as soon as I saw the questionAd_Astra_Per_Aspera wrote:thanks. Yup 300 is the answer .. could you explain how you are calculating the thee digits-7XX,X7X,XX7
7XX- 100 times
X7X=90 times
XX7-90 times
Also, were you able to do the question in under 2 mins ?
thanks
ok thats the trick..jkpimp168 wrote:im scared of these kind of questions because i always worry that my counts would overlap. like in this question, while 7X can appear 10 times (70-79), how can X7 still appear 9 times (17,27, 37...97) as "77" is overlapped!? am i thinking too much?
ooo...ook....i think i got it. so in "77", 7 appears 2 times, and 3 times in "777", yea? thx so much!tohellandback wrote:ok thats the trick..jkpimp168 wrote:im scared of these kind of questions because i always worry that my counts would overlap. like in this question, while 7X can appear 10 times (70-79), how can X7 still appear 9 times (17,27, 37...97) as "77" is overlapped!? am i thinking too much?
we are not being asked how many numbers..we are being asked how many times. so You gotta count those numbers twice:))
if you want to be on the safe side, try this:
numbers from 0-999 can be represented as XXX, 0<=x<=9
only one 7- like 7XX where x is any value from 0-9 except 7
3*9*9=243
two 7's=3*9=27. now each number contains two 7's. so total 7's=54:)
three 7's. only one number 777. But, three 7's
total 7's=243+54=3=300.
yepp!!!jkpimp168 wrote:ooo...ook....i think i got it. so in "77", 7 appears 2 times, and 3 times in "777", yea? thx so much!tohellandback wrote:ok thats the trick..jkpimp168 wrote:im scared of these kind of questions because i always worry that my counts would overlap. like in this question, while 7X can appear 10 times (70-79), how can X7 still appear 9 times (17,27, 37...97) as "77" is overlapped!? am i thinking too much?
we are not being asked how many numbers..we are being asked how many times. so You gotta count those numbers twice:))
if you want to be on the safe side, try this:
numbers from 0-999 can be represented as XXX, 0<=x<=9
only one 7- like 7XX where x is any value from 0-9 except 7
3*9*9=243
two 7's=3*9=27. now each number contains two 7's. so total 7's=54:)
three 7's. only one number 777. But, three 7's
total 7's=243+54=3=300.
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