>>153
を使って書くと、

str = 'apple'

(1..str.length).each do |i|
puts str[0...i] + "," + str[i..-1]
end

>>154
>puts str[0...1] + "," + str[0..-1]
>puts str[0...2] + "," + str[0..-2]

[0..-1], [0..-2]ではなく、[i..-1]だよ