Milestone Code Anomaly
A project tracker assigns codes to sequential milestones using three positions: letter, digit, letter. The intended rule is: the first and third letters each move forward two places in the alphabet, wrapping from Z to A; the digit moves forward three places modulo 10. The recorded sequence is: A1B, C4D, E7F, G0H, I3J, K6L, M9N, O9P. A colleague says there is exactly one transcription error in the list and that the next code after the corrected list is also needed for the audit.
1. Identify the transcription error and give the corrected code.
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Apply the rule to M9N: first letter M + 2 = O; digit 9 + 3 = 12, modulo 10 = 2; third letter N + 2 = P. The expected code is O2P, not O9P.
Check that M9N itself is consistent: K6L -> M9N is correct. So the single error is O9P, which should read O2P.
2. What is the next code after the corrected list?
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After O2P: O + 2 = Q; 2 + 3 = 5; P + 2 = R. The next code is Q5R.
The corrected sequence is: A1B, C4D, E7F, G0H, I3J, K6L, M9N, O2P, Q5R.
3. The colleague suggests the error could instead be in M9N. How would you test that claim?
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Test M9N against both neighbours. From K6L, the rule gives M9N, so M9N is consistent with the preceding code. From M9N, the rule gives O2P; the recorded O9P breaks the rule at the digit. If M9N were the error, the following code would also have to be wrong unless it happened to fit a different predecessor. Since exactly one error is claimed, the only consistent correction is O9P -> O2P.