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BrentGMATPrepNow
Bunuel
George takes 4 days to build one-third of a house, Michael takes 3 days to build one-sixth of the same house and Jose takes 5 days to build half of the same house. If all three men work together for 3 days and then George and Jose quit, how long will Michael take to build the remaining portion of the house alone?

A. 2 days
B. 5.1 days
C. 8 days
D. 7.5 days
E. 12 days

We can solve this question quite easily by assigning a "nice" value the job of building a house.
We want a value that works with all of the given numbers 4, 1/3, 3, 1/6, 5 and 1/2

Pro tip: 360 is often a good number to work with because it's divisible by so many other numbers
So let's say the job of building a house = hammering 360 nails

Let's first calculated each person's RATE

George takes 4 days to build one-third of a house
1/3 of a house = 1/3 of 360 = 120 nails.
If it takes George 4 days to hammer 120 nails, then George's rate = 30 nails per day

Michael takes 3 days to build one-sixth of the same house
1/6 of a house = 1/6 of 360 = 60 nails.
If it takes Michael 3 days to hammer 60 nails, Michael's rate = 20 nails per day

Jose takes 5 days to build half of the same house
1/2 of a house = 1/2 of 360 = 180 nails.
If it takes Jose 5 days to hammer 180 nails, Jose's rate = 36 nails per day

All three men work together for 3 days and . . .
The combined rate of all three men = 30 + 20 + 36 = 86 nails per day
So, in three days, the total number of nails hammered = (3)(86) = 258

The number of nails remaining to complete the job = 360 - 258 = 102

. . . then George and Jose quit, how long will Michael take to build the remaining portion of the house alone?
Michael (at his rate of 20 nails per day) must hammer the remaining 102 nails to complete the job.
Time = output/rate
So the time it takes Michael to complete the job = 102/20 = 5.1 days.

Answer: B

Great explanation BrentGMATPrepNow, could you show algebraic approach please as I don't quite understand how to calculalte for Michael's time here? Thanks Brent
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