Composition of Jupiter

Can anyone make sense of the numbers in this quote?

http://www.seds.org/nineplanets/nine...s/jupiter.html

Jupiter is about 90% hydrogen and 10% helium
(by numbers of atoms, 75/25% by mass) with
traces of methane, water, ammonia and "rock".

Hydrogen atoms have an atomic weight of about 1.
Helium atoms have an atomic weight of about 4.

This means the mass of hydrogen is 9/13 of the total, or 69%,
and the mass of helium is 4/13 of the total, or 31%.

Let's try something else.

There are two hydrogen atoms in a hydrogen molecule,
and one helium atom in a helium molecule. If Jupiter
were 90% hydrogen molecules and 10% helium molecules,
then the mass of hydrogen is 18/22 of the total, or 82%,
and the mass of helium is 4/22 of the total, or 18%.

-- Jeff, in Minneapolis

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