The average of $30$ numbers is $12$. The average of the first $20$ of them is $11$ and that of the next $9$ is $10$. The last number is:

The average age of four members of a family is $32$ years. If the age of a guest is included,then the average age increases by $12.5 \%$. What is the age (in years) of the guest?

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?

Average rainfall on Monday,Tuesday,Wednesday and Thursday is $420.5\, cm$ and average on Tuesday,Wednesday,Thursday and Friday is $440.5\, cm$. If the ratio of rainfall for Monday and Friday is $20:21$,find the rainfall in $cm$ on Monday and Friday.

The average marks of Sameer decreased by $1,$ when he replaced the subject in which he scored $40$ marks with two other subjects in which he scored $23$ and $25$ marks respectively. Later,he also included $57$ marks of Computer Science,and the average marks increased by $2.$ How many subjects were there initially?

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?