Discussion
Find the missing number :
12, 21, 23, 32, 34, ?
Answer: A
The missing number is 43.
The differences alternate between +9 and +2:
12 (+9) 21 (+2) 23 (+9) 32 (+2) 34 (+9) 43
Another way to see it: the pairs 12 and 21, 23 and 32, 34 and 43 are each a number and its digits reversed. 36 or 37 would add 2 or 3 again, and 42 does not continue either pattern.
Checking the rule on every step confirms it: 21 - 12 = 9, 23 - 21 = 2, 32 - 23 = 9, 34 - 32 = 2, so the next step is +9 again.
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