If 1 1 1 1 = R 2 2 2 2 = T 3 3 3 3 = E 4 4 4 4 = N Then 5 5 5 5 = ?

This is a digit-sum-to-word-ending code: take the row's repeated digit, add it to itself four times, spell the total as an English number word, and the LAST LETTER of that word is the code for that row.

1. 1 + 1 + 1 + 1 = 4, spelled FOUR, which ends in R — matching the code given for the first row.

2. 2 + 2 + 2 + 2 = 8, spelled EIGHT, which ends in T — matching the code given for the second row.

3. 3 + 3 + 3 + 3 = 12, spelled TWELVE, which ends in E — matching the code given for the third row.

4. 4 + 4 + 4 + 4 = 16, spelled SIXTEEN, which ends in N — matching the code given for the fourth row.

5. 5 + 5 + 5 + 5 = 20, spelled TWENTY, which ends in Y.

Every one of the four given rows (R, T, E, N) matches this same sum-then-last-letter rule exactly, with no exception, so applying it to the fifth row is fully justified rather than a guess.

So the code for 5 5 5 5 is Y.