The mechanic and his pupil need to make 420 details. The mechanic can implement this plan in 21 hour, and his pupil - in 42 hours. In how many hours will the mechanic and the pupil execute if work at the same time????

1) 430:21=20(ch) 2)420:42=10(ch) 3)20+10=30(ch) 4)420:30=14(ch) of Answer: 14 hours. 420/21=20 420/42=10 20+10=30 420/30=14 hours it is necessary to them together
Answer add