jon and jony can do a piece of work alone 12days and 24days respectively.they were working together but after a few days jon went away.then jony can do the
rest work in 3days.how many days are needed to complete the whole work?
jon does 1/12 per day
jony does 1/24 per day
together they do 1/12 + 1/24 = 1/8 per day
it took jony 3 days to finish
... 3/24 = 1/8
so together, they did 7/8 ... in 7 days
7 + 3 = 10 days for completion