A test was taken by $5$ students with scores $80$, $80$, $85$, $90$, $95$. What is the **weighted mean** computed as a sum of values weighted by frequency?

Weighted mean $= \dfrac{\sum (\text{value} \cdot \text{frequency})}{\sum \text{frequency}}$. Distinct values: $80$ (freq 2), $85$ (freq 1), $90$ (freq 1), $95$ (freq 1). $\dfrac{2 \cdot 80 + 1 \cdot 85 + 1 \cdot 90 + 1 \cdot 95}{2 + 1 + 1 + 1} = \dfrac{160 + 85 + 90 + 95}{5} = \dfrac{430}{5} = 86$. Matches the ordinary mean — which it should, since weighted mean with frequency weights = arithmetic mean. - Trap B is the **median**. - Trap A is the **mode** ($80$).