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 only two letters can be associated with digits for sure.