In a certain code language, 'P#ST' is coded as '4025' and 'RS#T' is coded as…

In a letter-to-digit coding puzzle, when a letter common to two given code-words sits at different positions in each word, a plain left-to-right positional decode is unreliable. Instead, match sets: a letter common to both code-words must correspond to a digit common to both codes, while the one letter present in only one word must correspond to the one digit present only in that word's code.

1. List the letters of both code-words: 'P#ST' contains P, #, S, T; 'RS#T' contains R, S, #, T.

2. The letters common to both words are #, S and T; the letter present in only one word is P (only in 'P#ST') and R (only in 'RS#T').

3. List the digits of both codes: '4025' gives 4, 0, 2, 5; '0125' gives 0, 1, 2, 5.

4. The digits common to both codes are 0, 2 and 5; the digit present in only one code is 4 (only in '4025') and 1 (only in '0125').

5. Since P is the letter unique to 'P#ST', it takes the digit unique to '4025', so P = 4; since R is the letter unique to 'RS#T', it takes the digit unique to '0125', so R = 1.

As a check, T sits at the same relative (last) position in both words and consistently carries the digit 5 in both codes, confirming the common-letter/common-digit pairing is internally consistent. A raw position-by-position match, by contrast, would wrongly give S two different digits, since S sits third in 'P#ST' but second in 'RS#T' — which is exactly why the unique-letter-to-unique-digit rule, not blind position, is the reliable method here.

So, the code for 'R' is 1.