If 1.5 men can pave 1.5 roads in 1.5 days, their rate is:

rate of 1.5 men = 1.5 roads/1.5 days = 1 road per day.

Therefore the rate of 6 men (4 groups of 1.5 men) is:

rate of 6 men= 1 road/day +1 road/day + 1 road/day + 1 road/day= 4 road/day

Therefore in 7 days, 6 men can build 7(4) roads:

7days(4roads/day)=28 roads