Question 857118
Since "Dave works twice as fast as Sam", this means that Sam takes twice as long. So if x = time for David to do the job alone, then 2x = time for Sam to do the job alone



1/x + 1/(2x) = 1/60



2/(2x) + 1/(2x) = 1/60



(2+1)/(2x) = 1/60



3/(2x) = 1/60



3*60 = 2x*1



180 = 2x



180/2 = x



90 = x



x = 90



2x = 2*90 = 180



So Dave takes 90 minutes and Sam takes 180 minutes