Posted by Kacey on Friday, May 1, 2009 at 2:05am.
A can paint a room in 3 hours, and B can paint a room in 4 hours. How long will it take if they paint it together?

math  RickP, Friday, May 1, 2009 at 3:10am
Use the equation
Tt = 1/Tf + 1/Ts
where Tt is the total time, Tf is the first person's time, and Ts is the second person's time. Now just plug and chug. 
math  Damon, Friday, May 1, 2009 at 6:36am
ah:
{ 1/Tt } = 1/Tf + 1/Ts
because
A > 1/3 rooms per hr
B > 1/4 rooms/hr
(A+B) > 1/3 + 1/4 rooms/hr = 7/12 rooms per hr
which is 12/7 hours/room 
math  benz, Wednesday, March 16, 2016 at 8:00am
7/12