Tiana can clear a football field of debris in 3 hours. jacob can clear the same field in 2 hours. if they decide to clear the field together, how long will it take them?

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 

solve for "t"

25 is obviously the correct answer