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by artstudent » Wed Jul 20, 2011 9:37 pm
Crew A can build a car in 20 minutes and Crew B can build a car in 25 minutes. If both crews work independently and start at the same time, what is the least amount of time required for the two crews to build 10 complete cars?

I had 111(1/9) minutes. OA is 120. Can someone explain?
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by pemdas » Wed Jul 20, 2011 10:40 pm
Crew A's speed 1/20
Crew B's speed 1/25
10 cars will be built in time T
>>>
1/20 - 1/25 = (5-4)/100 = 1/100 is the difference in speed between the crews A and B
hence 1 more (additional) car can be built by crew A than by crew B within 100 minutes

during first 100 minutes 5 cars can be built by crew A and 4 cars can be built by crew B + (Plus) within the next 20 minutes additional 1 (one car) can be built (completed by crew B. This makes T=100+20 or 120 minutes.

artstudent wrote:Crew A can build a car in 20 minutes and Crew B can build a car in 25 minutes. If both crews work independently and start at the same time, what is the least amount of time required for the two crews to build 10 complete cars?

I had 111(1/9) minutes. OA is 120. Can someone explain?
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by winniethepooh » Wed Jul 20, 2011 10:43 pm
As the work is done independently the combined work cannot be a fraction.

Consider first 100 hours of combined work:
Crew A makes 100/20 = 5 cars and Crew B makes 100/25 = 4 cars
Now 1 car has to be made so to take the least possible time and also prudently the car will be made by Crew A. Hence, additional 20 mins will be taken.
So, total = 100 + 20 mins = 120 mins.
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by amit2k9 » Thu Jul 21, 2011 1:39 am
In 100 mints A makes 5 and B makes 4 cars.
total = 9 cars made in 100 mints.

A takes less time to make 1 car = 20 min
hence 10th car will be made by A in 20 more mints.
total = 100+20 = 120 min
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by GMATGuruNY » Thu Jul 21, 2011 2:32 am
artstudent wrote:Crew A can build a car in 20 minutes and Crew B can build a car in 25 minutes. If both crews work independently and start at the same time, what is the least amount of time required for the two crews to build 10 complete cars?

I had 111(1/9) minutes. OA is 120. Can someone explain?
We could plug in the answers, which the GMAT would provide.
One answer likely would say 100 minutes.
In 100 minutes, A produces 100/20 = 5 cars and B produces 100/25 = 4 cars.
Total cars produced in 100 minutes = 5+4 = 9 cars.
The faster crew -- A -- will require 20 more minutes to produce the 10th car.
Total time = 100+20 = 120 minutes.
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