| Group | Size | Mean |
|---|---|---|
| A | 8 | 40 |
| B | 9 | 86 |
Combined mean = (n₁ × mean₁ + n₂ × mean₂) ÷ (n₁ + n₂) — weight by group size, never average the two means. The correct answer is 64.40. Traps to avoid: 61.60 comes from the "swapped weights" error; 126 comes from the "summed means" error; 86 comes from the "took larger group mean" error; 63.0 comes from the "unweighted mean" error.