I need to read one more time to understand why Mexicans and Canadias are not Americans. WTF :D America is the whole continent.

I really like this solution! For the sumDigits function, maybe Character::getNumericValue would work, too.

Outside the US, the word 'American" can refer to anyone from the Americas, i.e. Canadians, too. Usually no factor, though.

Well, your solution says you need 1 fold for an 0.0001m paper to reach 0.0001m. There's your problem ;-)

Simple tests contain only few examples of what you can expect, but when you hit submit, it's gonna test all edge cases (negative numbers, 0, big numbers) and throw many random tests, so passing sample tests is just 1 step towards completion :P

I still don't understand it this program works fine in normal "run simple test" but shows error when i try to submit it.

Always write what language you're using, so we can help.

Try printing the values and see if that helps, or post the input here, so we can tell you why it should return 4.

This isn't a suggestion, so I'm closing it, but you can still reply.

What every you comment me i can't see it it's hidden

Done, also completely rewrote the Java tests since the code was very crappy.

Please replace new Double() with Double.valueOf() to better comply with Java 11.