The average of $6$ numbers is $20$. If one number is removed, the average becomes $15$. What is the number removed?

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 of five numbers is $27$. If one number is excluded,the average becomes $25$. The excluded number is

The average age of $A, B, C, D$ five years ago was $45$ years. By including $x$,the present average age of all the five is $49$ years. The present age of $x$ is (in years):

The average age of $5$ sisters is $20 \, \text{years}$. If the age of the youngest sister is $4 \, \text{years}$,what was the average age of the group of sisters at the time of the birth of the youngest sister?

The average weight of five persons sitting in a boat is $38\, kg$. The average weight of the boat and the persons sitting in the boat is $52\, kg$. What is the weight of the boat (in $kg$)?