Question: Identify the last sequence in this series: AB3 BC5 CD7  ?
Answer: Look at the series closely. Let us write numbers equivalent to each alphabets, then observe these words:
Let us write each word and their equivalent numbers in a table as shown below:
A B 3 
B C 5 
C D 7 
? ? ? 
1 2 3 
2 3 5 
3 4 7 
? ? ? 
1 + 2 = 3 
2 + 3 = 5 
3 + 4 = 7 
4 + 5 = 9 



D E 9 
As we can see from the above table that sum of first two numbers are written at the last, and the first letter of each word is increasing by 1 alphabet.
Consider first word, which is AB3. In this case A is taken as 1 from the alphabet table and B is taken as 2 from the same table. While writing the word, we have written first to numbers in alphabets and last number as the sum of first two numbers.
In the next word we are increasing first two numbers by 1. So AB becomes BC and equivalent of BC from the alphabet table is 2 and 3. Finally we are adding 2 and 3 to arrive 5. So adopting the previous method of writing, the new word is BC5.
Same way CD7 and finally the new word would be DE and the number will be addition of equivalent of D and E from the alphabet table, which is the sum of 4 and 5 = 9. So the last word would be DE9.
