The average weight of $21$ boys was recorded as $64 \, kg$. If the weight of the teacher was added,the average increased by $1 \, kg$. What was the teacher's weight (in $, kg$)?

The average of $n$ numbers $x_{1}, x_{2}, \ldots, x_{n}$ is $\bar{x}$. Then,the value of $\sum_{i=1}^{n}(x_{i}-\bar{x})$ is equal to

The average salary per head of all workers of an institution is $Rs. 60$. The average salary per head of $12$ officers is $Rs. 400$. The average salary per head of the rest is $Rs. 56$. Then,the total number of workers in the institution is?

The average price of $80$ computers in an electronic shop is $Rs. 30,000$. If the highest and lowest price computers are sold out,then the average price of the remaining $78$ computers is $Rs. 29,500$. The cost of the highest price computer is $Rs. 80,000$. The cost of the lowest price computer is (in $Rs.$): (in $,000$)

The average expenditure of a man for the first five months is $Rs.\, 3600$ and for the next seven months it is $Rs.\, 3900$. If he saves $Rs.\, 8700$ during the year,his average income per month is (in $Rs.$)

The average of prime numbers between $1$ and $20$ is