Working together, Bob and Andrew make    of the job per hour.


Working alone, Bob makes    of the job per hour.


Hence, working alone, Andrew makes   -  =  =   of the job per hour.


It means that Andrew needs   hours =  hours = 6 hours and 40 minutes to complete the job working alone.    Answer

    1)  You can solve this problem/(such problems) even without using equations.

        Freely manipulating with fractions is enough.


    2)  Use the notion "rate of work" and remember:

            a)  when two persons work together, their combined rate of work is the sum of individual rates;


            b)  when the combined rate of two workers is given along with the rate of one of them, the rate 
                of the other worker is the difference of combined rate and the given individual rate.