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?
Answer: B
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