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Since there is only 5 ⋅ 5 = 2 5 possible remainders that a pair of consecutive Fibonacci numbers can have upon division by 5 , the sequence of remainders must become periodic with period ≤ 2 5 .
So we can check the remainders of the Fibonacci numbers upon division by 5, which is as follows;
1 , 1 , 2 , 3 , 0 , 3 , 3 , 1 , 4 , 0 , 4 , 4 , 3 , 2 , 0 , 2 , 2 , 4 , 1 , 0 , 1 , 1 , . . . .
We find that the period is 2 0 . Hence
F 2 0 1 6 + F 2 0 1 7 ≡ F 1 6 + F 1 7 ≡ 2 + 2 ≡ 4 ( m o d 5 ) .