Let the required number of days be x. Less persons, More days (Indirect Proportion)

More working hours per day, Less days (Indirect Proportion)

Persons 30 : 39 :: 12 : x

Working hours/day 6 : 5

30 x 6 x x = 39 x 5 x 12

x = (39 x 5 x 12)/(30 x 6)

x = 13