To do this you have to divide 3/4 by 1/6. To do this multiply 3/4 by the reciprocal of 1/6. The reciprocal of 1/6 is 6/1. So you have to multiply 3/4 and 6/1. This will give you 18/4. You now simplify it to 4 2/4, which can be simplified even more to 4 1/2. It will take Trevor 4 1/2 days to paint 3/4 of the fence.
BTW- In case you are wondering how long it'll would take him to paint the entire fence, it would be 6 days.