In a certain code language, 'RAIN' is written as '182915' and 'SUN' i…

1  2    3    4   5    6   7     8    9  10  11  12  13

 A   B   C    D   E    F   G    H    I    J    K    L   M

 ↓    ↓    ↓    ↓    ↓     ↓   ↓    ↓     ↓   ↓    ↓    ↓    ↓

 Z   Y    X   W   V    U   T    S    R   Q   P    O   N

26  25  24  23  22  21  20  19  18  17  16  15  14

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THE LOGIC FOLLOWED HERE IS:

IST LETTER OF GIVEN WORD ALPHABET POSITIONAL VALUE REMAINS SAME

THE ALTERNATE LETTER OF GIVEN WORD ALPHABET  POSITIONAL VALUE +1

AS

R → ( SAME POSITIONAL VALUE )  → 18

A  → ( +1 )  → 2

I → ( SAME POSITIONAL VALUE )  → 9

N  → (+1)  → 15

AND

S → ( SAME POSITIONAL VALUE )  → 18

U  → ( +1 )  → 22

N → ( SAME POSITIONAL VALUE )  → 14

SIMILARLY

M → ( SAME POSITIONAL VALUE )  → 13

O  → ( +1 )  → 16

O → ( SAME POSITIONAL VALUE )  → 15

N  → (+1)  → 15