Dropquest 2012, Step 1

detect_password.rb

# The product of the first two digits is 24.
# The fourth digit is half of the second digit.
# The sum of the last two digits is the same as the sum of the first and third digits.
# The sum of all the digits is 26.
# The second and third digits add up to the last digit.

def number_to_digits(n)
  n.to_s.split("").map(&:to_i)
end

def product_of_1st_2_digits_is_24?(digits)
  digits[0] * digits[1] == 24
end

def the_4th_digit_is_half_the_2nd_digit?(digits)
  digits[3] == digits[1] / 2
end

def sum_last_2_digits_is_eql_sum_1st_and_third?(digits)
  digits[3] + digits[4] == digits[0] + digits[2]
end

def sum_is_26?(digits)
  digits.inject(:+) == 26
end

def the_2nd_and_3rd_digits_add_up_to_last_digit?(digits)
  digits[1] + digits[2] == digits[4]
end

def is_password?(n)
  digits = number_to_digits(n)

  product_of_1st_2_digits_is_24?(digits) && the_4th_digit_is_half_the_2nd_digit?(digits) &&
  sum_last_2_digits_is_eql_sum_1st_and_third?(digits) && sum_is_26?(digits) &&
  the_2nd_and_3rd_digits_add_up_to_last_digit?(digits)	
end

puts (9999...100000).each.detect &method(:is_password?)