4X+9( 1/6-X)= 1
X= 1/ 10
Then it takes 10 weeks for A to do it alone. B it takes 15 weeks. It's time to consider options.
Let's set a weekly salary of x, and the weekly salary is (5.2/6-X).
4X+9(5.2/6-X)=4.8
X=0.6
Then A needs 6W. B it needs 4W. Save money and consider B.