Randy can rebuild an automobile engine in a hours. Alvin can rebuild the same engine in b hours. If Randy and Alvin work together at their respective rates to rebuild the engine, which of the following represents the portion of the job that Randy will not have to complete? Choose the correct choice from the following answers.
A) ab/(a+b)  B) (a-b)/(a+b)  C) b/(b-a)  D) a/(a+b)  E) b/(a+b)
Let total work completion be denoted as 1.   Randy and Alvin can together complete work in ab/a+b hours.   Randy's 1hr work is 1/a.   Alvin's 1 hr work is 1/b.   Alvin's portion of total work = Alvin's 1hr work /total 1 hr work.   =(1/b)/((a+b)/ab)   =a/(a+b)   That's the portion of work Randy need not do.   ANSWER:a/(a+b)
<result>D</result>
Randy can rebuild an automobile engine in a hours. Alvin can rebuild the same engine in b hours. If Randy and Alvin work together at their respective rates to rebuild the engine, which of the following represents the portion of the job that Randy will not have to complete?