A and One

a) hundred, thousand and million cannot be used alone;
Common mistake: They want to live for hundred years.

the words can be used in the singular with a (informal style) or
one (speaking precisely).
Example:
They want to live for a hundred years.
The car journey took exactly one hundred hours.
I’ve saved a thousand dollars.
Pay the inland revenue board of taxes one thousand dollars only.
Mix one pint of water with one pound of flour.

b) A is only used at the beginning of a number;
Example:
156 a hundred and fifty-six
3,156 three thousand, one hundred and fifty-six