2 Atoms of Hydrogen in a Molecule of Water
This is a Ditloid. The answer is 26 Miles in a Marathon. (approximately)
13 M of N = 13 moons of neptune.
568 M in a P = 568 millilitres in a pint.
4 musicians in a quartet
Millimetres in a Yard
The answer is '39 Inches in a Metre'.
15 Men on a Dead Mans Chest
10 Centuries in a Millenium
metric absolute zero
60 Minutes in a Degree
This is a Ditloid. The answer is 535 Members of Congress.
7 Ages of Man (Shakespeare)