VIAMC 2025 Mock Test — Level 6 (Senior) Question 9

Dũng can solve many types of Rubik's cubes. During training with a $3\times3$ cube, he records the times for 25 consecutive solves in the table. The time intervals in seconds are $[8;10),[10;12),[12;14),[14;16),[16;18)$, with respective frequencies 4, 6, 8, 4, 3. Which option is closest to the standard deviation of this grouped data? Editorial grading convention: use the school grouped-data standard deviation with divisor n=25; the alternative divisor n−1=24 is compared in the solution.

The following source mathematical images preserve the printed option/formula/figure variants; text and choices above or below provide clean typesetting.