When Mary paints a house, it takes her 4 hours. When Lisa joins Mary, and they work together, it takes them only 3 hours to paint a house of the same size. How long would it take for Lisa to paint a house of the same size by herself?  Choices:
A) 5 hr  B) 6 hr  C) 7 hr  D) 12 hr  E) 20 hr
Here, the rate equation becomes:   (# of houses) = (painting rate) x (time)   When Mary paints a house, it takes her 4 hours. Thus, (1 house) = (Mary’s rate) x (4 hr), so her rate is 1/4.   When Mary &amp; Lisa paint together, it takes 3 hrs. Thus, (1 house) = (combined rate) x (3 hr) and the combined rate = 1/3.   To find a combined rate, we add individual rates.   (combined rate) = (Mary’s rate) + (Lisa’s rate)   1/3 = 1/4 + (Lisa’s rate)   (Lisa’s rate) = 1/3-1/4=1/3*4/4-1/4*3/3=4/12-3/12=1/12   Lisa’s rate is 1/12 of a house every hour, or in other words, 1 house in 12 hrs. Thus, it would take her 12 hours to paint a house of the same size.   Answer: 12 hr.
<result>D</result>
When Mary paints a house, it takes her 4 hours. When Lisa joins Mary, and they work together, it takes them only 3 hours to paint a house of the same size. How long would it take for Lisa to paint a house of the same size by herself?