Bob can type 2 letters in 4 hours while bill can do it in 6. how many hours would it take them, working together, to type six letters?

Bob's production rate is given by:
 r1 = 2/4 = 1/2 letter / hour
 The production rate of Bill is given by:
 r2 = 2/6 = 1/3 letter / hour
 Thus, the production rate of both is:
 r1 + r2 = 1/2 + 1/3 = 5/6 letter / hour
 Then, the production of 6 letters is given by:
 rate * time = letters
 (5/6) t = 6
 t = (6/5) 6
 t = 36/5 = 7 1/5 hours = 7hr 12 min
 
 it would take them, working together, to type six letters about:
 7hr 12 min

The answer to the question is d