(2) 1/a； 1/b； 1/a+ 1/b； 2( 1/a+ 1/b)； 1-2( 1/a+ 1/b)；

Example 1 (1) Solution: It will take X days to set up two teams to finish this project.

( 1/ 18+ 1/ 15)X = 1

1 1/90X= 1

X=90/ 1 1