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In the pattern 1,2,3,4,5 all the numbers are consecutive. To continue the pattern, we add the next consecutive number from five; six.

Lee, after writing seventeen digits, got to 13. My method was this: From 1 to 9, each number has one digit. So from seventeen digits, we take away 9, so we get 8. Each number from 9 onwards has 2 digits. So we divide 8 by 2, which gives us 4. So we add 9 and 4, which gives us 13.

1-9=9 digits.
17-9=8
8÷2=4
9+4=13

We repeat the same method for getting to 20, but backwards; we’re trying to find the total digits instead of the number.

20-9=11
11x2=22 digits
9 digits+22 digits=31 digits

So the answer is 31 digits from 1 to 20.