B it takes y weeks to complete it alone, and it costs b million yuan a week.
Judging from the meaning of the question:
1/x+ 1/y= 1/6
4/x+9/y= 1 (the specific step of solving the equation is omitted)
∴x= 10
y= 15
6a+6b=5.2
4a+9b=4.8
∴a=0.6
b=4/ 15
∴ A alone requires10 * 0.6 = 60,000.
B alone requires 15 * 4/ 15 = 40000.
Choose company B.