Since Christina can make the entire job in 3 hours, working alone, she makes    of the job per hour.


Since Victoria can make the entire job in 5 hours, working alone, she makes    of the job per hour.


Working together, the two girls make   +  of the job per hour.


Let  t be the time (in hours) for the two girls to complete the job, working together.

Then they make    part of the job per hour.


It gives you an equation


     +  = .


At this point the setup is completed: the equation is constructed, and your next step is to solve it.


For it, multiply both sides by 15t = 3*5*t to rid off the denominators. You will get


    5t + 3t = 15.


Simplify and solve for t :


    8t = 15,

    t = 15/8 =  hours = 1 hour and 52.5 minutes.    ANSWER