Solution: Because it takes 20 hours for Mr. Zhang to complete the sorting, Mr. Zhang sorts all 1/20 every hour.

Because it takes 65,438+02 hours for Mr. Liu to complete the sorting alone, Mr. Li sorts all 65,438+0/65,438+02 every hour.

First, Mr. Zhang worked alone for four hours, and * * did it all (1/20)*4= 1/5, leaving 1- 1/5=4/5.

Two people do it together, and sort out all1/20+112 = 2/15 every hour.

So it takes (4/5)/(2/ 15)=6 (hours) to sort out the rest.

Therefore, * * * needs 4+6= 10 (hour).