Jonathan can type a 20 page document in 50 minutes, Susan can type it in 30 minutes, and Jack can type it in 24 minutes. Working together, how much time will it take them to type the same document?  Choose the correct choice from the following answers:
A) 5 minutes B) 11 minutes C) 15 minutes D) 18 minutes E) 20 minutes

Rationale: you may set up common equation like this : job / a + job / b + job / c = job / x memorize this universal formula , you will need it definitely for gmat . and find x from this equation in this specific case , the equation will look like this : 20 / 50 + 20 / 30 + 20 / 24 = 20 / x if you solve this equation , you get the same answer b ( 11 )
Answer: B