is a 4-digit number which, when multiplied by 995, yields a product that ends in 2015.
What is sum of all such possible ?
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 .
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This is equivalent to solving for:
9 9 5 × S ≡ 2 0 1 5 ( m o d 1 0 0 0 0 )
This will give 5 solutions spaced 2000 apart for S with S ∈ [ 0 , 1 0 0 0 0 ] since the gcd of 995 and 10000 is 5. We can find the solution under 2000 by solving for S in
9 9 5 S + 1 0 0 0 0 y = 2 0 1 5 ⇒ 1 9 9 S + 2 0 0 0 y = 4 0 3
Solve the LDE for 1 9 9 S + 2 0 0 0 y = 1 - we get by standard methods that S = 1 7 9 9 works. Hence, one possible value of S is 1 7 9 9 × 4 0 3 ( m o d 2 0 0 0 ) = 9 9 7 .
Thus the 4-digit values of S are 2 9 9 7 , 4 9 9 7 , 6 9 9 7 , 8 9 9 7 which have a sum of 2 3 9 8 8 .