Question 23027
Solution: As given in problem 
                            In 5 hours Jake can do 1 job
                           =>In 1 hour he can do 1/5 job
                           similarly 
                            in 3 hours Jerry can do 1 job
                           => in 1 hour she can do 1/3 job
                          so together in 1 hour they can do (1/5) +(1/3)job
                                                            =(3+5)/15job
                                                            =8/15job            Now, jack has done 2 hours job alone before she          joins                him.
so job done by jack alone=2/5job
job remaining=1-(2/5)=3/5
this 3/5job has to be done by both of them
by unitary method,
as 8/15 job is done in 1 hour

    1 job is done in 15/8 hours
   3/5 job is done in (15/8)*(3/5)hours
                       9/8 hours
                       ie 1hour7.5minutes. 
so they will complete job in 1hour and 7.5 minutes