I agree with pessimist - that this is just a trick question.
However, if it's just worded badly and when you said "half a hole", you actually meant a hole half the size/depth/volume (or whatever) of the original holes (assuming the original holes were all the same size), then .....
6 men take 6 days to dig holes
i.e. each man digs one hole in 6 days
... assuming it takes half the time to dig "half a hole" ....
then it would take 1 man 3 days to dig "half a hole".