Which is bigger? 5/13 or 6/15

An advertising firm has an old computer that can prepare a whole mailing in 6 hours. with the of newer model the job is complete in 2 hours. how long would it take the newer model to do the job alone?

What is the solution to the equation? k/6.4=8.7 2.3 5.568 15.1 55.68

Whats are the trickiest steps to do when adding or subtracting rational expressions? how does knowledge of rational numbers u in adding and subtracting rational expressions?