Question 823903
If only 1 of the men, then the work rate is {{{(1/12)}}} of the group of 12 men.  This means, 1 man goes at the rate of {{{1/(27*12)}}} houses per hour.  The question is about the rate for 9 men:


Knowing their rates are additive, {{{9(1/(27*12))=1/36}}} houses per hour, which using the reciprocal, is 36 hours to paint one house.