Alen and Border can do a work individually in 21 and 42 days respectively. In how many days they can complete the work, working alternatively?

Alen's one day's work =1/21
Border's one day's work = 1/42
Alen and Border's two days work(working alternatively) = (1/21) + (1/42) = 1/14
So, Alen and Border do 1/14 work in 2 days
So, they complete the work in 14 x 2 = 28 days.