My apologies, this is only tangentially related to Haskell and I am not looking for a solution. I am confused with the assignment itself and hence asking here. The assignment (https://www.seas.upenn.edu/~cis194/spring13/hw/01-intro.pdf) says: ====== BEGIN Double the value of every second digit beginning from the right. That is, the last digit is unchanged; the second-to-last digit is dou- bled; the third-to-last digit is unchanged; and so on. For example, [1,3,8,6] becomes [2,3,16,6] Add the digits of the doubled values and the undoubled dig- its from the original number. For example, [2,3,16,6] becomes 2+3+1+6+6 = 18 ======== END Firstly, I am confused as to how the doubled values are being added to the undoubled number in the above example. It looks like only the individual numbers of the doubled values are being added Secondly, later on in the assignment: ======== BEGIN Example : validate 4012888888881881 = True Example : validate 4012888888881882 = False ========= END If we are to follow the algorithm described (double the value of "every second digit" beginning from the right, last digit unchanged", then the above numbers are identical EXCEPT For the last digits (1 and 2) Any help is appreciated in decoding these instructions. Again please do not provide Haskell code, since I want to attempt to write it myself. I am just looking for help with the algorithm. Regards,