B : 25 :: E : ? Which number completes the analogy?

The letter is answered by its place counted from the far end instead of the near one. B stands 2nd from the left, and counting back from Z it stands 25th, since 26 − 2 + 1 = 25. E stands 5th from the left, so from the far end it stands 26 − 5 + 1 = 22. The trap 21 is what a solver gets by subtracting 5 from 26 and forgetting to add the 1 back; 5 is simply E's own place, and 23 fits nothing.