One printing press can print an order of advertising brochures in 3.5 hr. Another press can do the same job in 5.5 hr. How long will it take if both presses are​ used?

Answer 1

(1/3.5) + (1/5.5) = 1/T

2/7 + 2/11 = 1/T

T = 77/36 hr

Why?

The numbers given are rates to do the whole job!