worst practice, eval can run any arbitrary code

But for this isn't ok or should we do this code regarding that security?

Don't use eval!

This is clever! However, I recently read that the eval() function is a big security risk and is not advised under any circumstances anymore: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#never_use_eval!

Fortunatelly we are wikipedia gifted :-) Read the first paragraph and "Change of base":
https://en.wikipedia.org/wiki/Logarithm

At the end logarithm to the base of 100% - evap_per_day (in %) of thershold (in %) gives you the number of days.

I do think the author do this on purpose to simulate a unclear requirement problem faced everyday by programmers.

And if you points it out, he will pull the ole "xxx numbers of people passed the kata" crap. Boils anyones blood really.

I cant do most of his kata but I dont hate him.

Change language version to 2.4/ES6.

Beautiful! Can someone explain what is happening for someone not mathematically gifted?

Im in awe at the size of this code