Mean of $30$ values was found to be $20$. Later it was discovered that one value was wrongly taken as $30$…
Mean of $30$ values was found to be $20$. Later it was discovered that one value was wrongly taken as $30$ instead of $40$. What is the correct mean (to two decimals)?
$19.67$
$20.33$
$20.50$
$21.00$
Solution
Wrong sum $= 30 \times 20 = 600$. Correct sum $= 600 - 30 + 40 = 610$. Correct mean $= \dfrac{610}{30} \approx 20.33$.