I think sum and average can be better, (BTW, other methods are excellent!)

def sum
reduce(0, :+)
end

def average
empty? ? NaN : sum / size # As description says average() on an empty array must return NaN
end

This solution is not good enough. If there is an empty array, sum method will return nil, i think it's better to return 0. Also average method will lead to error if size is 0.

[].reduce(:+) => nil

0 / 0 => error