The average of certain first consecutive natural numbers is $20.5$. Find their sum.

The average of $13$ results is $60$. If the average of the first $7$ results is $59$ and that of the last $7$ results is $61$,what will be the seventh result?

The average monthly expenditure of a family for the first four months is $Rs.\, 2570$,for the next three months $Rs.\, 2490$ and for the last five months $Rs.\, 3030$. If the family saves $Rs.\, 5320$ during the whole year,the average monthly income of the family during the year is (in $Rs.$)

The average weight of $8$ persons increases by $2.5\, kg$ when a new person comes in place of one of them weighing $65\, kg$. What is the weight of the new person? (in $kg$)

$A$ group of boys has an average weight of $36 \,kg$. One boy weighing $42 \,kg$ leaves the group and another boy weighing $30 \,kg$ joins the group. If the average now becomes $35.7 \,kg$,then how many boys are there in the group?

In a family,the average age of father and mother is $38 \text{ years}$,whereas the average age of father,mother,and their only daughter is $28 \text{ years}$. Then,the age of the daughter is? (in $\text{years}$)