It is clever, but not the best solution as no one understands with a brief look what's going on.

I believe in that case kata would be higher than 7

Thx. I believe it could be refactored even more, but have no time for this...
By the way, you may just press the buttons "Best Practices" and "Clever". Usually, comments are used to ask something or add some information.

"Useless fuffery!" is amazing and it shows us how much guys just copy-paste solutions))

U needn't to use Math.floor before use type casting from double to int.

https://www.codewars.com/kata/reviews/564852b6ba9517dd1c00006d/groups/620e6bb00e603c00014efb20

Picture to explain:
https://www.educative.io/api/edpresso/shot/5763976954642432/image/5733201030938624

assertEquals(30, Kata.cost(1), "For input 45");
Must be like that I believe:
assertEquals(30, Kata.cost(1), "For input 1");

Hardcoding))

I love such ways where programmer rewrite functions to simplier way just cause of simplifier formula))

He did it beautifuuuul)))

Also don't know...

You are on right way. Just read this:
https://database.guide/how-log-works-in-postgresql/

It's really strange that it works. Cause of long casted to int may return wrong number or even negative number.