math
posted by Kacey .
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?

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. 
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 
7/12
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