Analogy: 215:474::537:?

Concept: In a number analogy of the form A : B :: C : ?, one common rule is a digit-sum invariant — the sum of every digit across both numbers in a pair stays the same for every pair in the analogy. Once that constant total is known from the given pair, the missing number must have whatever digit sum keeps the total unchanged.

1. Digit sum of 215 = 2 + 1 + 5 = 8.

2. Digit sum of 474 = 4 + 7 + 4 = 15.

3. Combined total for the first pair = 8 + 15 = 23.

4. Digit sum of 537 = 5 + 3 + 7 = 15.

5. For the second pair to keep the same total of 23, the missing number's digit sum must be 23 − 15 = 8.

Cross-check: Checking every option's digit sum against this requirement — 26 gives 2 + 6 = 8, matching exactly, while 27 gives 9, 25 gives 7, and 22 gives 4, none of which equal 8.

Hence, 26 correctly completes the analogy 215 : 474 :: 537 : ?.