Learn English – Should I use ‘a’ or ‘an’ before numbers

I'm British, but I can answer for the UK and US:

$100 — a hundred dollars or one hundred dollars?

"A hundred dollars" is how I'd say it in speech. "One hundred dollars" is how I'd write it on a cheque.

$201 — two hundred [and?] one dollar[s?]?

In the UK we'd say "two hundred and one dollars". In the US, they might say "two hundred one dollar[s]".

$1500 — fifteen hundred dollars or one thousand five hundred dollars?

"Fifteen hundred dollars" is how I'd say it in speech. The more "proper" way to say it, and the way I'd write it on a cheque is: "One thousand, five hundred dollars" (never "one thousand and five hundred dollars").

$1525 — fifteen twenty-five dollars or [one/a] thousand five hundred twenty-five dollars?

I'd never say "fifteen twenty-five dollars", I'd either say "Fifteen hundred and twenty five dollars", or "one thousand, five hundred and twenty five dollars". Americans might skip the "and".

For a non-English speaking country, say the number fully using "one", for the sake of clarity. In some countries though (such as the Netherlands and Norway), the use of "fifteen hundred" etc. is the same in that language too.

There are different uses of the noun "reason" here.

B,C,D) For these it is a countable noun.

I have reasons for doing this.

Note the use of the plural. This looks very similar to (A) but is using "reason" as a countable noun.

0) Another use of "reason" is an uncountable noun.

There is reason in madness.

A) Example (A) is a phrase "to have reason". Here "reason" means that what you do is reasonable, you could justify it. It is from the uncountable noun. So what you do can be justified.

I have reason to believe you.

I have good reason to do this.

Or you could justify the actions of someone else.

He has reason to do that.

Sometimes, you can justify your expectation of something even if it is not true or provably true,

He has reason to know.