Question

Twelve men take 6 hours to finish a piece of work. After the 12 men have worked for 1 hour, the contractor decides to call in 8 more men so that the work can be completed earlier. How many more hours would 20 men take to complete the remaining work?

Answer 1

Twelve men take 6 hours to finish a piece of work.
In 1 hour, the 12 men will finish 1/6 of the work
( 1 job ) / ( 6 hrs ) = ( 1/6 of a job ) / ( 1 hr )
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That means there is of the job
left to do.
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If 12 men take 6 hours to finish a piece of work,
1 man will take 12 times as long, to finish the work, so
( 1 job ) / ( 6*12 = 72 hrs ) = rate of working for 1 man, or

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After 1 hour, there are a total of men working
20 men will work 20 times as fast as 1 man on the
of a job that remains, so
Let = hours for the 20 men to finish the job