The average weight of $4$ men $A, B, C$ and $D,$ is $67\, kg.$ The $5^{th}$ man $E$ is included and the average weight decreases by $2 \,kg$. A is replaced by $F.$ The weight of $F$ is $4 \,kg$ more than $E$. Average weight deereases because of the replacement of $A$ and now the average weight is $64\, kg.$ Find the weight of $A ?$

The average age of $5$ members is $21 \,years.$ If the age of the youngest member be $5 \,years,$ find out the average age of the family at the birth of the youngest member.(in $years$)

The average weight of $36$ students is $50 \,kg$. If was found later that the weight of one of the student was misread as $73 \,kg$. whereas his actual weight was $37 \,kg$. Find the correct average. (in $kg$)

A student finds the average of ten $2 -$digit numbers. While copying numbers, by mistake, he writes one number with its digits interchaged. As a result his answer is $1.8$ less than the correct answer. The diference of the digits of the number, in which he made mistake, is

The average pocket-money of $3$ friends $A, B, C$ is $Rs.\, 80$ $month.$ If $B$ spends double and $C$ spends triple of whate $A$ spends during that month and if the average of their unspent pocketemoney is $Rs.$ $60,$ then $A$ spends (in $Rs.$)

If the mean of $a, b, c$ is $M$ and $a b+b c+c a=0,$ then the mean of $a^{2}, b^{2}, c^{2}$ is