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This question appears on my daughter's Mathematics class in secondary school:

"There are 100 rooms in a hotel. Joe must number the rooms from 1 to 100 with plastic digits. How many times does he need the digit 7?"

My answer is: 07 17 27 37 47 57 67 77 87 97

Number 7 will appear 11 times. However, the book's answer on the back is: 20 times Can you explain why??

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    $\begingroup$ $70, 71, 72, \cdots , 79$ $\endgroup$ – Simon S Nov 7 '15 at 20:26
  • $\begingroup$ OMG! You right. I feel sooo stupid now. How I could omit them? $\endgroup$ – Adam Nov 7 '15 at 20:52
  • $\begingroup$ We can close this topic now I guess. thanks a lot. $\endgroup$ – Adam Nov 8 '15 at 21:10
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    $\begingroup$ Probably not a hard problem, but definitely a great father. $\endgroup$ – linear_combinatori_probabi Apr 4 '18 at 14:32
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07,17,27,37,47,57,67,70,71,72,73,74,75,76,77,77,78,79,87,97

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7,17,27,37,47,57,67,70,71,72,73,74,75,76,77,78,79,87,97 So the answer is 20

(previous answer said 19, probably omitting the two 7s that are present in '77')

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