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Work problem

by oks » Fri Jun 12, 2009 4:50 pm
Crew A can build a complete car in 20 minutes and Crew B can build a complete car in 25 minutes. If both crews work independently and start at the same time, what is the shortest amount of time it oculd take for 10 cars to be complete?

A. 111 1/9 min
B. 115 2/5 min
C. 116 min
D. 120 min
E. 125 min

OA is D - could someone explain please? Maybe I am just missing the point. Thanks.
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Re: Work problem

by Naruto » Fri Jun 12, 2009 6:49 pm
oks wrote:Crew A can build a complete car in 20 minutes and Crew B can build a complete car in 25 minutes. If both crews work independently and start at the same time, what is the shortest amount of time it oculd take for 10 cars to be complete?

A. 111 1/9 min
B. 115 2/5 min
C. 116 min
D. 120 min
E. 125 min

OA is D - could someone explain please? Maybe I am just missing the point. Thanks.
A simple approach would be to take LCM of 20 and 25 mins which is 100 mins. Now in 100 mins each crew independently will complete 5 and 4 cars respectively, makin it a sum of 9. Now the question is to complete 10 cars in shortest possible time, 9 are done in 100 mins, the first crew will take less time to complete a car compared to the crew who takes 25 mins. so 10 cars can be done in 100+20 mins i.e 120 which is the answer. Hope this is clear.
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Re: Work problem

by ssmiles08 » Fri Jun 12, 2009 7:04 pm
Naruto wrote:
oks wrote:Crew A can build a complete car in 20 minutes and Crew B can build a complete car in 25 minutes. If both crews work independently and start at the same time, what is the shortest amount of time it oculd take for 10 cars to be complete?

A. 111 1/9 min
B. 115 2/5 min
C. 116 min
D. 120 min
E. 125 min

OA is D - could someone explain please? Maybe I am just missing the point. Thanks.
A simple approach would be to take LCM of 20 and 25 mins which is 100 mins. Now in 100 mins each crew independently will complete 5 and 4 cars respectively, makin it a sum of 9. Now the question is to complete 10 cars in shortest possible time, 9 are done in 100 mins, the first crew will take less time to complete a car compared to the crew who takes 25 mins. so 10 cars can be done in 100+20 mins i.e 120 which is the answer. Hope this is clear.
Good approach Naruto!
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by tohellandback » Fri Jun 12, 2009 7:50 pm
lets do it like this
crew A takes 20 minutes
crew B takes 25 minutes
if both had taken 20 minutes, 10 cars would have been ready in 100 minutes
but it is not the case. so lets say how far they go in 100 minutes
in 100 minutes
A makes 5 cars
B makes 4 cars
now 1 car is left
A takes 20 min, B takes 25. min is 20 min
so total time=100+20 =120 mins
The powers of two are bloody impolite!!
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