so Joe can mow the 6 acres in 5 hours, whilst Sara can do it in only 4 hours. So in after 1 hour of working, Joe has only done 1/5 of the whole thing, and after working for 1 hour as well, Sara has however done 1/4 of the whole job.
let's say they both work together and do it in "t" hours.
now, after one hour has passed, Joe has done 1/5 of the whole thing, and Sara has done 1/4 of it, and since the job took "t" hours, after 1 hour it has only being done 1/t thus far, thus

which is 2 hours 13 minutes and about 20 seconds.