Question 888134
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 {{{ 1 - 1/6 = 5/6 }}} 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
{{{ 1/72 }}}
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After 1 hour, there are a total of {{{ 12 + 8 = 20 }}} men working
20 men will work 20 times as fast as 1 man on the {{{ 5/6 }}}
of a job that remains, so
Let {{{ t }}} = hours for the 20 men to finish the job
{{{ 20*( 1/72 ) = (( 5/6 )) / t }}}
{{{ 20/72 = 5/(6t) }}}
{{{ 5/18 = 5/( 6t) }}}
Multiply both sides by {{{ 18t }}}
{{{ 5t = 15 }}}
{{{ 3 }}}
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It will takes the 20 men 3 more hours to 
complete the remaining work
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Hope I got it