4.In a stack there are 5 books each of thickness 20 mm and 5 paper sheets each of thickness 0.016 mm. What is the total thickness of the stack?

We have the thickness of books = $$5 \times 20 = 100 mm$$
Also, the thickness of 5 paper sheets = $$5 \times 0.016 mm = 0.080 mm$$.
Total thickness of the stack = $$100 mm + 0.080 mm = 100.080 mm = 1.0008 \times 10^2 mm$$