To find the total thickness of the stack, we need to add up the thicknesses of the books and the paper sheets.

Thickness of the books: 7 books x 25 mm/book = 175 mm

Thickness of the paper sheets: 7 sheets x 0.020 mm/sheet = 0.14 mm

Total thickness of the stack: 175 mm (books) + 0.14 mm (paper sheets) = 175.14 mm

Therefore, the total thickness of the stack is 175.14 mm.