Volume:
Let S the area need to paint,so in every liter:
A paint area of S/40
B paint area of (S/40)*(3/4)=3S/160:
The requirement volume of B is 160/3 L.
Time:
A cost 100H.
B cost (1+1/3)100 = 400/3 H
Using Paint A:
The cost of 40L A is 240.00
The cost of labor of 100H is 3600.00
Total cost 3840.00
Using Paint B:
The cost of 160/3 L B is 240.00
The cost of labor of 400/3H is 4800.00
Total cost 5040.00
Paint A is cheape