When Mary paints a house, it takes her 4 hours. When Lisa joins Mary, and they work together, it takes them only 3 hours

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Vincen: Legendary Member; Posts: 2898; Joined: Thu Sep 07, 2017 2:49 pmFollowed by: 5 members; Thanked: 6 times

by Vincen » Tue Apr 28, 2020 8:04 am

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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?

A. 5 hr
B. 6 hr
C. 7 hr
D. 12 hr
E. 20 hr

[spoiler]OA=D[/spoiler]

Source: Magoosh

Source: Beat The GMAT — Problem Solving | Tue Apr 28, 2020 8:04 am

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Re: When Mary paints a house, it takes her 4 hours. When Lisa joins Mary, and they work together, it takes them only 3 h

by Scott@TargetTestPrep » Fri May 01, 2020 3:26 am

Vincen wrote: ↑
Tue Apr 28, 2020 8:04 am
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?

A. 5 hr
B. 6 hr
C. 7 hr
D. 12 hr
E. 20 hr

[spoiler]OA=D[/spoiler]

Source: Magoosh

Mary’s rate is 1/4, and Lisa’s rate is 1/L. We can create the equation:

1/4 + 1/L = 1/3

Multiplying by 12L, we have:

3L + 12 = 4L

12 = L

Answer: D

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