The average weight of $36$ students is $50 \,kg$. It was found later that the figure of $37 \,kg$ was misread as $73 \,kg$. What is the correct average? (in $kg$)

In a family,the average age of a father and a mother is $35 \text{ years}$. The average age of the father,mother,and their only son is $27 \text{ years}$. What is the age of the son (in $\text{years}$)?

The average marks obtained by $180$ students in an examination is $50$. If the average marks of passed students is $80$ and the average marks of failed students is $40$,then what is the number of students who failed the examination?

The average earning of a mechanic for the first four days of a week is $Rs. 18$ and for the last four days is $Rs. 22$. If he earns $Rs. 20$ on the fourth day,his average earning for the whole week is (in $Rs.$)

In a coconut grove,$(x+2)$ trees yield $60$ nuts per year,$x$ trees yield $120$ nuts per year,and $(x-2)$ trees yield $180$ nuts per year. If the average yield per year per tree is $100$,find the value of $x$.

The average weight of $A, B$ and $C$ is $84 \, kg$. If $D$ joins,the average weight now is $80 \, kg$. If another person $E$,who is $3 \, kg$ heavier than $D$,replaces $A$,then the average weight of $B, C, D$ and $E$ becomes $79 \, kg$. What is the weight of $A$ in $kg$?