Question 723508
Use rates as number of hectares per hour.  


8 workers plough 6/5 hectare per hour.  1 worker has the rate of {{{6/(8*5)}}} hectare per hour.


The question is about 5 workers.  This group goes at the rate of {{{5*(6/(8*5))}}} hectare per hour.  {{{highlight(5*(6/(8*5))=3/4)}}} hectare per hour.  So for  this group to plough 9 hectare, 
{{{highlight(9/(3/4)=12)}}} hours.