Answer with Explanation:

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   }

Therefore,30 x 6 x x = 39 x 5 x 12

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

=> x = 13.