Randy can rebuild an automobile engine in b hours. Alvin can rebuild the same engine in a 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 Alvin will have to complete? Choose the most appropriate option:
A) ab/(a+b)  B) (b)/(a+b)  C) b/(b-a)  D) a/(a+b)  E) b/(a-b)
Randy can do the job in a hrs, Alvin can do the job in b hrs.   When they work together, you need to find the fraction of work that Randy doesn't need to do i.e. the fraction that will be done by Alvin.   Ratio of Randy's speed:Alvin's speed = b:a (since time taken by them is in the ratio a:b)   So Alvin does b/(a+b) of the work   Answer ((b)/(a+b)).
<result>B</result>
Randy can rebuild an automobile engine in b hours. Alvin can rebuild the same engine in a 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 Alvin will have to complete?