This took me some time to figure out, too. Basically a new Array is being defined with a given length, and it's an empty array. Then this empty array is being joined, with a given letter joining the empty indexes. For example, for i = 2, the Array has 3 empty indexes: [,,] and then it is joined by whatever letter is in letters[i]. Say letters[2] equals the letter a. Then if we join the empty array together with the letter in between each empty index, you get "aa".

I think it's unclear how to calculate the percentage of candles that will fall to the floor. In your example, the string y is 'abc' and has a value of 198. How do you calculate the percentage from 198? But then again I'm totally new, maybe I'm missing something?