42 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?
Choose one:  A) 10  B) 16  C) 13  D) 14  E) 19

Rationale: "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 : 42 : : 12 : x working hours / day 6 : 5 30 x 6 x x = 42 x 5 x 12 x = ( 42 x 5 x 12 ) / ( 30 x 6 ) x = 14 answer d"
Answer: D