Question

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?

Answer 1

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.

Answer 2

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

Answer 3

7/12