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

Answer:

it would take them, working together, to type six letters а bout:

7hr 12 min