Thank you, Abbe! Good points. I added an extra test case line to the bottom of my description, and I now explain that you need to create 2 to_a methods, a class and instance method. Yes, it's useful to implement basic data structures from scratch, instead of depending on a library/gem. That fact that Ruby doesn't have a built-in linked list is bad for programmers but good for us learners.

I'm rayning3 on Skype, if you want to talk. If you have any questions about Ruby, I've been programming in it for over 1 year now. My email is rayning@gmail.com. I'm happy to help! What's your new job? Congratulations.

C'mon, Kasare12. You shouldn't use a gem (someone else's code) to make primes. Figure out an algorithm to create your own. If you are using a gem, you can't modify the code internally. Also, this problem goes up to a big number, on purpose. The basic brute force way of finding primes won't work. You need to find a faster, more efficient way. Project Euler (https://projecteuler.net/) does the same thing. They always ask you to find something that may be simple if done for a small case. But then you must solve it for a 100 digit number (or more). Brute force usually won't work.

Sure, Kasare12. How may I see your code to check it?

Same here. So frustrating. I keep passing the first 5 tests but failing the last 2. Can someone give us a hint about the tree size in the last 2 tests, or what the final test results for the last 2 tests should look like? I am stuck.

Problem should say what the max primes are that it tests up to.