Question 395590
Let s = # of hours it takes the son to do the work alone.

Then {{{3/9  + 4.5(1/9  + 1/s) = 1}}}
<==> {{{7.5/9 + 4.5/s = 1}}}
<==> {{{4.5/s = 1.5/9}}} ==> s = 27, after cross multiplication.