Because a stands for 97 in ascii code.

who knows? maybe it's op grandma age?
https://stackoverflow.com/questions/47882/what-is-a-magic-number-and-why-is-it-bad

Because 'a'.charCodeAt = 97 , b - 98, and so on

Only exclude numbers with the digit "5".
There is nothing said about 10. :-)
Look at the 2nd example in the description.

And you have also to exclude numbers like 51, 52, 53 and so on...

And how could I help you?

1175 people did this kata without any problems.

So maybe you should read it again.

Or tell me, what exactly do you not unterstand.