The average age of 20 boys in as class is 12 years. 5 new boys are admitted to the class, whose average age is 7 years, the average age of the boys in the class becomes?
A. 8 years B. 9 years C. 10 years D. 11 years
Correct Answer: C
Explanation
Using the formula sum \(=\bar{x} n\) to form a 3-column table, we have \begin{array}{|l|l|l|} \hline \(\bar{x}\) & \(n\) & Sum \\ \hline 12 & 20 & 240 \\ \hline 7 & 5 & 35 \\ \hline \(\bar{x}\) & 25 & 275 \\ \hline \\ \(\overline{ x }=\frac{\text { sum }}{n}=\frac{275}{25}\) \\ \(\overline{ x }=11\) years \end{array}
