Digits 1 to 8 are mapped to 3 letters each, 9 is mapped to only 2. 
(3 × 8) + 2 = 26 which is the total number of letters in English. 
Each letter is mapped to a unique code, but remember that each number is mapped
to 2323 letters. 
It is best to start from the small words. Let us attack ‘is’ and ‘as’ first. 
IS = 35, AS = 56. S = 5 , I = 3 , A = 6. 
Now, THE is 458. OF = 79. PEACOCK does not have a ‘7’ in in. It has an O but no F, so F should be
7 and O should be 9. 
THE has 458, DESIGNATED has T and E but no H. Is there a number in THE but not in
DESIGNATED? 
H should be 4. T and E should be 5 and 8 in some order. 
Now let us look at INDIA. INDIA = 13366. I = 3 and A = 6. So, N and D should be 1 and 6 in some
order. BIRD has a D, BIRD = 1334. 
What does this mean? This tells us D = 1 and N = 6. 
Let us have another look at T and E. T and E and 5 and 8 in some order. Is there a word that has
only one of T & E but not both? 
NATIONAL has T but not E. NATIONAL has 8 but not 5. BINGO! We know T has to be 8 and E has
to be 5. 
Now let us go word by word. PEACOCK = P56C9CK. So, PCCK should be 8899. 9 is allotted only
to two numbers. 
We know that O = 9. So, what can we say about 9? 
If C takes 8, then both P and K becomes 9 which is not possible as 9 can be assigned to only two
letters. 
Therefore C has to take 9 and P , K takes the value 8. 
DESIGNATED = 1553G66851. The missing number should be G. Or, G = 7. 
NATIONAL = 6683966L. The missing number should be L. Or, L should be 1. 
BIRD = B3R1. 3 and 4 are missing. B and R should be 3 and 4 in some order. 
B and R occur only once each in this sequence, so there is no way of resolving this.

From the above inferences, we can see that L takes 1.