Printer B takes X min to print 40 pages. Per min. it can print 40/X pages.
So Printer A will take X+4 mins. to print 40 pages. Per min. it can print 40/(X+4) pages.
Given that together they can print 50 pages in 6 mins.
So 6(40/X + 40/X+4) = 50
=> 240X + 960 + 240X = 50X^2 + 200X
=> 50X^2 + 280X - 960 = 0
=> 5X^2 + 28X - 960 = 0
=> X = -2.4 which cannot be taken as X cannot be -ve and X = 8
Printer A takes 8+4 = 12 mins. to print 40 pages,
For 80 mins it will take = 2*12 = 24 mins.
your answer is right and I followed your method. anyway it took me more than 2 minutes. is there any shortcut? i.e. if a spends 4 minutes more than b to make 40 copies, it would spend 1 more minute to make 10 more copies so 10/x + 10/(x-1) = 50/6
let's divide both members by 10 and get 1/x + 1/(x-1) = 5/6 which makes things much easier. any other solutions?