The average of the first $9$ integral multiples of $3$ is

What is the average of all numbers between $100$ and $200$ which are divisible by $13$ (in $.5$)?

$A$ batsman in his $17^{th}$ innings makes a score of $85$ runs,and thereby,increases his average by $3$ runs. What is his average after the $17^{th}$ innings? He had never been 'not out'.

In an examination,the average marks obtained by the girls of a class is $85$ and the average marks obtained by the boys of the same class is $87$. If the girls and boys are in the ratio $4:5$,the average marks of the whole class (approx.) is closest to:

Three years ago,the average age of a family of $5$ members was $27 \, \text{years}$. The present average age is also $27 \, \text{years}$ after the inclusion of a child into the family. Find the age of the child in years.

In an examination,the average marks were found to be $60$. The marks of $80$ candidates had to be changed from $95$ to $70$ each,and the average came down to $55$ marks. The total number of candidates who attended the examination was: