Instead of the three dots, write a digit to make the fraction reducible. (Find all possible cases.) 6 ... 5/1 ... 5

Any digit will make the fraction reducible, since both numerator and denominator numbers end in 5. A factor of 5 can always be removed. If your ... is restricted to a single digit, that digit can be any of {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}.