Odd One Out — In the twin-like pairs (919, 949), (646, 686), (828, 848), (434, 464), identify which pair does not obey “middle digit doubled” while first and last stay fixed.

Introduction / Context:
Three pairs hold outer digits constant while doubling the middle digit on the right value; one pair violates this. Detecting a consistent digit operation is essential.

Given Data / Assumptions:

• We compare left and right numbers digit-wise: left = x y z → right = x (2y) z.
• Doubling means numeric doubling (e.g., 3 → 6; 4 → 8).

Concept / Approach:
Check whether the right number keeps the same first and last digits and doubles the middle digit of the left number.

Step-by-Step Solution:
646 → 686: outer digits 6 and 6 fixed; middle 4 → 8 (doubled) ✓828 → 848: outer digits 8 and 8 fixed; middle 2 → 4 (doubled) ✓434 → 464: outer digits 4 and 4 fixed; middle 3 → 6 (doubled) ✓919 → 949: outer digits 9 and 9 fixed; middle 1 → 4 (quadrupled, not doubled) ✗

Verification / Alternative check:
There is no carry effect because all doubled middles (2→4, 3→6, 4→8) remain single-digit. Only 1→4 fails the doubling criterion (should be 2).

Why Other Options Are Wrong:

• (646, 686): follows the doubling rule.
• (828, 848): follows the doubling rule.
• (434, 464): follows the doubling rule.

Common Pitfalls:
Reading speed errors (confusing 2→4 with 1→4). Always track digit-by-digit.

Final Answer:
919, 949