There are the same number of books on the two shelves, that is, 1 10 books; So the previous level was 120, and the next level was 100.

Method 1:

220÷2= 1 10 (Ben)

Upper layer:110+10 =120 (Ben)

Subordinate:110-10 =100 (Ben)

Method 2:

Upper layer-10= lower layer+10.

Upper limit-lower limit =20

Upper layer: (220+20)÷2= 120 (Ben)

Lower floor: (220-20)÷2= 100 (Ben)