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?

Let the required number of days be x.

Less persons, More days (Indirect Proportion)

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

$Persons30:39Working hours/day6:5⋮⋮ 12:x$

=> 30 * 6 * x = 39 * 5 * 12 

=> x= 13