Captain is coded as fbfnbrb then humble is coded as

ppzdxj Explanation- C is the 3rd alphabet. 3*2=6 and 6th alphabet is f. So C becomes f. And likewise. If an alphabet is the 25th alphabet ie Y then 25*2 will be 50. The sequence will repeat in itself. So 50-26=24 and hence Y will become X. You can follow this pattern for each and every alphabet in the word. A is first alphabet. 1*2=2 and 2nd alphabet is B so A got replaced with B. P is 16th letter. 16*2=32. 32-26(Because the total number of alphabets is 26 and if the number is greater than this it will repeat the cycle) = 6 and 6th alphabet is F. So P got replaced by F.

Please provide the logic