The analysis assumes that it takes X days to complete the project, and the total workload of 40 workers in X days should be equal to the total workload of 52 workers in X+6 days. After technological innovation, the work efficiency of workers has increased by 50%, which is equivalent to 1 worker to the original 1.5 worker, so it is not difficult to list the equation.

It took x days to finish this project, but the original plan was x+6 days. Because the work efficiency has increased by 50%, there are

52(x+6)=40( 1+50%)？ x，

52x+3 12=60x，

X = 39 (days).

A: It took 39 days to complete this project.