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We call a number memorable if every digit in the number is either a 1 or adjacent to a 1 For example, the numbers 111,81,101   and 1771 are all memorable, but the numbers  12321,786,140,  and 1999 are not memorable.

How many positive 3-digit numbers are memorable?

 

What it means is that the number has to be touching a 1 in the digits.

 Dec 19, 2019
 #1
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We can break into cases:

 

111 - 1 number
11* - 10 numbers
1*1 - 10 numbers
*11 - 10 numbers
*1* - 100 numbers

 

This gives us a total of 131 memorable numbers.

 Dec 19, 2019
 #2
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I tried it, and it didn't work.

I think the problem is that, you're overlapping a few numbers

Guest Dec 19, 2019
edited by Guest  Dec 19, 2019
 #3
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See the answer here:   https://web2.0calc.com/questions/help_27513

Guest Dec 19, 2019

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