First, lets determine how many holes/day 6 people can dig:

8holes/3.5days=(16/7) holes/day

This means that 1 person can dig:

(1/6)(16/7)=(8/21) holes/day

This means that 4 people can dig:

4(8/21)=(32/21)holes/day

Now let's determine how many days it would take the 4 guys to dig 10 holes:

10holes (21/32)day/hole=6.5625 days

This can be rounded up to 7 days.