Obviously cannot check all of them

How many integers n n are there in the interval 1 n 2015 1\leq n\leq2015 such that n n and n n n^n have the same last digit?


The answer is 1208.

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1 solution

Manuel Kahayon
Dec 29, 2015

There are quite a few cases for this.

If n n ends in 1, 0, 5, 6 or 9 then n n n^n always ends in the same ones digit as n n . Since there are 5 such occurrences of such numbers from 1 to 10, then

no. of occurrences from 1-2010 = 5 ( 2010 10 10 + 1 ) = 1005 5(\frac {2010-10} {10} +1)=1005

Therefore, no. of occurrences of such from 1-2015 is 1007 (2011 and 2015 included.)

If n n ends in 3 or 7, then n n n^n ends in the same ones digit as n n if and only if n 13 ( m o d 20 ) n \equiv 13 \pmod {20} or n 17 ( m o d 20 ) n \equiv 17 \pmod {20} . Since there are 2 occurrences of such numbers from 1-20, therefore

no. of occurrences from 1-2000 = 2 ( 2000 20 20 + 1 ) = 200 2(\frac {2000-20} {20} +1)=200

Therefore, no. of occurrences of such from 1-2015 is 201 (2013 included.)

If n n ends in either 2, 4 or 8, then n n n^n will never end in the same digit as n n . As such, there are 0 occurrences of n n which ends in any of there numbers.

Therefore, our answer is 1007 + 201 = 1208 1007 + 201 = 1208

Yes Exactly same way.

Kushagra Sahni - 5 years, 5 months ago

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