39 persons can repair a road in 12 days, working 5 hours a day. In how many days will 30 persons, working 6 hours a day, complete the work?

Since 39 persons can repair a road by working 5 hours in a day in 12 days.
∴ 1 persons can repair a road by working 5 hours in a day in 39 x 12 days.
∴ 1 persons can repair a road by working 1 hours in a day in 39 x 12 x 5 days.
∴ 30 persons can repair a road by working 1 hours in a day in 39 x 12 x 5 / 30 days.
∴ 30 persons can repair a road by working 6 hours in a day in 39 x 12 x 5 / 30 x 6 days.
∴ 30 persons can repair a road by working 6 hours in a day in 39 x 2 x 1 / 6 days.
∴ 30 persons can repair a road by working 6 hours in a day in 39 x 1 x 1 / 3 days.
∴ 30 persons can repair a road by working 6 hours in a day in 13 days.