=( 1-20/40)/50= 1/ 100

1/( 1/ 100)= 100

It takes 100 days for Team B to finish alone.

2)

x/40+y/ 100= 1

y= 100-(5/2)x

y & lt70， 100-(5/2)x & lt； 70，x & gt 12

12 & lt； x & lt 19

x= 14， 16， 18

***3 sets of reasonable schemes to choose from.

3) x= 14, y=65, and the project cost =14 *1.5+65 *1= 86.

X= 16, y=60, and the project cost =16 *1.5+60 *1= 84.

X= 18, y=55, and the project cost =18 *1.5+55 *1= 82.

For the scheme with the least project cost, the construction period of team A is 18 days, and that of team B is 55 days.