This a4 paper measures 297 by 210 mm, and is marked '80 grams per square metre'. what is the best estimate of the weight of one sheet?

297 mm x 210 mm = 62370mm^2  (square mm)

62370/1,000,000=0.06237m^2   (square metre)

0.06237m^2 x 80 g/m^2 = 4.9896 g

Therefore, the A4 paper weighs approximately 4.9896 grams

* you can round up to 5 grams though.

in that passage i would say the sentence which best describes the overall impact of the setting is when they mention the absence of people because it suggests that taking a walk at night is unusual.