so Tiana can do it in 3hrs, so, in 1hr she has only done 1/3 of the work, in 3 hours she's done 3/3 of the work or a "whole" or 1, and thus the whole work
now, Jacob can do it in 2hrs, so in 1hr he has done 1/2 of the total work
now, what if they do it together? well, in 1 hr, Tiana would have done 1/3 of the work, whilst Jacob would have done 1/2 of the work, working together, they'd have done.. hmmm let's say "t" is the hours it takes to finish for both working together, so in 1hr, they had done 1/t
thus [tex]\bf \cfrac{1}{3}+\cfrac{1}{2}=\cfrac{1}{t}[/tex]
solve for "t"
