See the explanation below.
Work Step by Step
If a mismatch were to occur, then 50% of the copies of the DNA-strand would be mutate. If a mismatch is repaired, there will be a 50% chance that the mutated strand is used and a 50% chance that the original strand will be used as the template for the repairing process. Therefore, there would be the same amount of mutations as if no repairing goes on at all.