1, choose two teams with fewer days to do it alone; That is, choose a and b to do it together.

Need1÷ (110+112) = 60/11day.

2. Comparison:

(1) Party A and Party C need to work together-1÷ (110+1/20) = 20/3 days;

A gets 20/3× 18 = 1.2 million yuan; C got 20/3× 8 =160/3 ≈ 533,000 yuan; Total cost = 120+53.3 = 1.73 million yuan.

(2) Joint operation requirements of Party B and Party C-1÷ (112+1/20) =15/2 days;

B get15/2×15 =1125000 yuan; C gets15/2× 8 = 600,000 yuan. Total cost =112.5+60 =1725,000 yuan;

(3) If Party A cooperates with Party B, Party A will get 60/1×18 = 982,000 yuan;

2. 60/11×15 = 818200 yuan. The total cost is 98.2+81.82 =1800200 yuan.

So choosing the combination of B and C is the most economical.