9 is the wrong answer because you should have 10 hooks.

One corner of your curtain is hanging unsupported.

I absolutely agree with you, this situation also causes misunderstanding :(

and then in the second example where the length of the curtain is 180
[0-20][20-40][40-60][60-80][80-100][100-120][120-140][140-160][160-180]
the maximum permissible length between the hooks is 22, and I have only 20. Why then 9 is the wrong answer, where did 17 come from?

and please note I don't measure anything, I just fold the curtain in half ((((

You're not crazy. But how are you going to measure 45? You can't, not in this kata.

You can only get 112.5 from 225 without measuring.

This is just bizarre. The first test expects a result of 9 on a test of numberOfHooks(225, 45) ??
45 is a factor of 225.

Consider brackets ([]) as the external hooks and pipes (|) as inner hooks.
[0 - 45 | 45 - 90 | 90 - 135 | 135 - 180 | 180 - 225 ]

Count 'em. 6.

Am I crazy or is this kata nuts?

The hell is going on here?