How many integers are there in the interval such that and have the same last digit?
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There are quite a few cases for this.
If n ends in 1, 0, 5, 6 or 9 then n n always ends in the same ones digit as n . Since there are 5 such occurrences of such numbers from 1 to 10, then
no. of occurrences from 1-2010 = 5 ( 1 0 2 0 1 0 − 1 0 + 1 ) = 1 0 0 5
Therefore, no. of occurrences of such from 1-2015 is 1007 (2011 and 2015 included.)
If n ends in 3 or 7, then n n ends in the same ones digit as n if and only if n ≡ 1 3 ( m o d 2 0 ) or n ≡ 1 7 ( m o d 2 0 ) . Since there are 2 occurrences of such numbers from 1-20, therefore
no. of occurrences from 1-2000 = 2 ( 2 0 2 0 0 0 − 2 0 + 1 ) = 2 0 0
Therefore, no. of occurrences of such from 1-2015 is 201 (2013 included.)
If n ends in either 2, 4 or 8, then n n will never end in the same digit as n . As such, there are 0 occurrences of n which ends in any of there numbers.
Therefore, our answer is 1 0 0 7 + 2 0 1 = 1 2 0 8