The average of $17$ numbers is $10.9$. If the average of the first $9$ numbers is $10.5$ and the average of the last $9$ numbers is $11.4$, find the middle number.

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 $, kg$)?

$30$ pens and $75$ pencils altogether were purchased for $Rs. \, 510$. If the average price of $75$ pencils was $Rs. \, 2$,then what was the average price of a pen? (in $Rs.$)

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

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 of $6$ observations is $12$. $A$ new seventh observation is included and the new average is decreased by $1$. The seventh observation is