Means Evan completes the 1/168 of the job each hour, and together they complete 1/56 each hour. 1/56 = 3/168, so Evan completes 1/168 of the job each hour and Emma completes the other 2/168 (so together they add up to be 3/168). So Emma completes 2/168 = 1/84 of the job per hour if she does it alone. So 84 hour is how long it takes.
