get the mean value of an array of integers but skip outliers

def mean(numbers)
    return 0 if numbers.empty?
    return numbers.first if numbers.length == 1
    return numbers.inject(:+) / numbers.length
end

mean([2, 3, 4, 5]) # should return 3.5
mean([2, 3, 4, 5, 100]) # should return 4.0