Credit Card Validation

[yonderson]

The Luhn algorithm for validating credit cards

1. Take orginal number: 377956570944726

2. Reverse number: 627449075659773

3. Double every second number: _4_8_18_14_12_18_14

4. Add numbers up*: 6+4+7+8+4+9+0+5+5+3+5+9+7+5+3
*If more than nine, subtract nine

Answer = 80. (Valid if divisible by ten)