It takes one machine 4 hours to complete a production order and another machine 9 hours to complete the same order. How many hours would it take both amhcines working simultaneously at their respective rates to complete the order?  Choose the correct choice
A) a) 7/12  B) b) 2 10/13  C) c) 1 5/7  D) d) 3 1/2  E) e) 7
RT = W   Given:   R = 1/9 + 1/4 = 13/36   T = ?   W = 1   T = 1 / (13/36) = 36/13 = 2 10/13   Answer: b) 2 10/13
<result>B</result>
It takes one machine 4 hours to complete a production order and another machine 9 hours to complete the same order. How many hours would it take both amhcines working simultaneously at their respective rates to complete the order?