H D Y S M W N B Q P O C R T B L Z V E G U F
Which is the only letter that occurs twice ?
Answer: A
Correct answer: B
Reading through H D Y S M W N B Q P O C R T B L Z V E G U F, the letter B appears twice - at the eighth position and again at the fifteenth.
The series has twenty-two letters and every other letter appears once, so B is the only repeat. Working systematically rather than by impression is what makes this quick: a single pass marking each letter as it is met finds the duplicate without rereading.
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