This solves the HackThisSite 'basic' mission number 6.

HTSBasic-6.rb

# Enter the encoded password from HackThisSite
encoded_password = "a9;96j6i"

# empty string for the decoded password
password = ""

# Loop for the length of the encoded password
0.upto(encoded_password.length-1) do |x|
	password += ((encoded_password[x].ord)-x).chr
end

# Print the decoded password
puts "Original: #{encoded_password}"
puts "Decoded: #{password}"