Answer:
8 hours
Step-by-step explanation:
A&D can clean a house in 8 hours together.
Proportionally, Denise works at x. Andrea works at 4x. There is a total of 5x.
Out of the 5x, Denise accounts for x.
x/5x = 1/5 of the work over 8 hours.
1/5 ( Denise's time for cleaning the whole house) = 8 hours.
Multiply both sides by 5:
5(1/5)(Denise's time for cleaning the whole house) = 5*8
Denise's time for cleaning the whole house = 40 hours
I like this method better...
That means in ONE hour, they get 1/8 of the job done.
Let's say Denise takes x hours to clean the house.
In one hour, she would do 1/x of the job (x can't be 0 hours).
Since Andrea does the house 4 times as fast as Denise, who takes x hours to do the job, Andrea takes x/4 hours.
In one hour, Andrea does 4/x of the job.
In one hour the part of the job Andrea does, plus the part of the job Denise does, must total 1/8.
(4/x) + (1/x) = 1/8
5/x = 1/8
Cross multiply:
x = 40
So it would take Denise 40 hours to clean the house by herself. (She's not really that efficient, or the house was REALLY dirty.
Andrea takes x/4 hours or 40/4 = 10 hours to clean the house by herself.
So with a little help from Denise, Andrea decreases her time to clean the house from the 10 hours it would take by herself to 8 hours.
