Question 813537
Keep everything in terms of hours
{{{ 5*16 = 80 }}} hrs
The rate of working of 9 men is:
( 1 job ) / ( 80 hrs )
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Twice as many men can do the same 
job in half the time, so
18 men can do 1 job in {{{ 80/2 = 40 }}} hrs
If they work 5 hrs a day, 
{{{ 40/5 = 8 }}}
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18 men can do the job in 8 days