prp  qrp  prr  pqr  qrp  rpr  qqr  rpr  qpr  qrr qpr  qpr

If every third letter starting from the left is replaced by a number beginning with 1 then which letter/ number will be 16th from the right?

Here "mba" is repeated 

 mbambambambambam

Here the letter series is alternative reverse order series. So we get P, N.

The difference of the two consecutive numbers are 1, 4, 9, 16, 25, 35. 

This is clearly , +12, +22, +32, +42, +52,+62 

Here 35 is wrong, So our wrong number is 91

First difference is 6 and second is 12. But if we take third as 18, then fourth difference will be 24. Then 93 can not be the last term.

So we have to take the difference as 6, 12, 24 and 48. 

So Answer is 21+24 = 45 

The first digit is formed by adding the numbers which are the results of powers of 2 increased by one and second letter is the corresponding alphabet to that number...last number is the next number to the first one...therefore

15=7+8 (i.e. 2^3)

O is corresponding alphabet to 15

and 16 comes after 15 

$$\begin{gathered} b w x y z a \\ d s t u v c \\ f o p q r e \end{gathered}$$

Given series follows Cubes on Prime Numbers 

i.e 23, 33, 53,...but  93is 1331

so 1381 is wrong