What is the average of the first $10$ even numbers?

The average salary per head of all the workers in a company is $Rs. \,95$. The average salary of $15$ officers is $Rs. \,525$ and the average salary per head of the rest is $Rs. \,85$. Find the total number of workers in the workshop.

The sum of five numbers is $260$. The average of the first two numbers is $30$ and the average of the last two numbers is $70$. What is the third number?

The average marks of a class of $35$ children is $35$. The marks of one of the students,who got $35$,was incorrectly entered as $65$. What is the correct average of the class?

The average age of Donald,his wife,and their two children is $23 \text{ years}$. His wife is $4 \text{ years}$ younger than Donald,and his wife was $24 \text{ years}$ old when his daughter was born. He was $32 \text{ years}$ old when his son was born. What is the average age of Donald and his daughter? (in $\text{years}$)

The average age of $24$ boys and their teacher is $15 \text{ years}$. When the teacher's age is excluded,the average age decreases by $1 \text{ year}$. What is the age of the teacher in years?