The average weight of a group of $20$ boys was calculated to be $89.4 \text{ kg}$ and it was later discovered that one weight was misread as $78 \text{ kg}$ instead of the correct one of $87 \text{ kg}$. The correct average weight is in $\text{kg}$.

The average marks of Sameer decreased by $1,$ when he replaced the subject in which he scored $40$ marks with two other subjects in which he scored $23$ and $25$ marks respectively. Later,he also included $57$ marks of Computer Science,and the average marks increased by $2.$ How many subjects were there initially?

The average of five consecutive odd integers is $27$. What is the product of the first and the last number?

The arithmetic mean of the scores of a group of students in a test was $52$. The brightest $20\%$ of them secured a mean score of $80$ and the dullest $25\%$ a mean score of $31$. The mean score of the remaining $55\%$ is

The average age of $30$ girls is $13$ years. The average age of the first $18$ girls is $15$ years. Find the average age of the remaining $12$ girls.

The average of $6$ numbers is $8$. What is the $7^{th}$ number so that the average becomes $10$?