End in 2015, part 2

S S is a 4-digit number which, when multiplied by 995, yields a product that ends in 2015.

What is sum of all such possible S S ?

Example : 2916 is a number which when multiplied by 107, yields 312012, which ends in 2012.


This question is from the set starts, ends, never ends in 2015 .


The answer is 23988.

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1 solution

Dylan Pentland
Nov 3, 2015

This is equivalent to solving for:

995 × S 2015 ( m o d 10000 ) 995\times S \equiv 2015 \pmod{10000}

This will give 5 solutions spaced 2000 apart for S S with S [ 0 , 10000 ] S\in [0,10000] since the gcd of 995 and 10000 is 5. We can find the solution under 2000 by solving for S S in

995 S + 10000 y = 2015 199 S + 2000 y = 403 995S+10000y=2015 \Rightarrow 199S+2000y=403

Solve the LDE for 199 S + 2000 y = 1 199S+2000y=1 - we get by standard methods that S = 1799 S=1799 works. Hence, one possible value of S S is 1799 × 403 ( m o d 2000 ) = 997 1799 \times 403 \pmod{2000}= 997 .

Thus the 4-digit values of S S are 2997 , 4997 , 6997 , 8997 2997,4997,6997,8997 which have a sum of 23988 23988 .

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