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Workings

Hint
One inch = 25.4 mm.

Answer
Millimetres in a mile.

Reasoning
One inch = 25.4 mm.

So:

1 mile = 1760 yards = 1760 x 3 feet = 1760 x 3 x 12 inches = 1760 x 3 x 12 x 25.4 millimetres = 1,609,344 mm in 1 mile.

And:

2 kilometres = 2000 metres = 2000 x 100 centimetre = 2000 x 100 x 10 millimetres = 2000 x 100 x 10 ÷ 25.4 inches = 2000 x 100 x 10 ÷ 25.4 x 16 = sixteenths of an inch ≈ 1,259,843 sixteenths of an inch in 2 kilometres.

Workings

Hint
Think about it carefully.

Answer
A hole, or a shadow.

Workings

Hint
The next-to-last letters are: N, N, E, I, L, O, R, E, U, U.

Answers
-s-e-d = ascend -p-a-g = sprang -r-f-r = prefer -y-r-d = hybrid -r-f-e = trifle -a-b-n = carbon -a-e-a = camera -m-z-s = amazes -s-f-l = useful -i-s-e = tissue

Workings

Hint
The first in the list would be Hydrogen.

Answer
Fluorine.

These are the elements in atomic number order.