What is the average of first $7$ multiples of $7$?

The average temperature of Monday,Tuesday,Wednesday and Thursday is $60^{\circ}$. The average temperature for Tuesday,Wednesday,Thursday and Friday is $63^{\circ}$. If the ratio of temperature for Monday and Friday is $21: 25,$ then what is the temperature of Friday? (in $^{\circ}$)

The average age of $30$ girls is $13$ years. The average age of the first $18$ girls is $15$ years. Find the average age of the remaining $12$ girls.

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 age of $8$ members of a group is $40 \text{ years}$. The age of $1$ member is $55 \text{ years}$. After his retirement,a new member of age $39 \text{ years}$ joins the group. What is the effect of this replacement on the average age of the group?

The average of $3$ numbers is $22$. If the first number is $\frac{3}{8}$ of the sum of the other two numbers,what is the first number?