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 $Rs.$)

The average age of $x$ and $y$ is $20$ $years.$ If $z$ were to replace $x,$ the average would be $19$ and if $z$ were to replace $y$, the average would be $21.$ Find the ages of $x, y$ and $z$.

The average of husband, wife and their child $3\, years$ ago was $27\, years$ and that of wife and the child $5\, years$ ago was $20\, years.$ The present age of the husband is (in $years$)

$16$ children are to be divided into two groups $A$ and $B$ of $10$ and $6$ children. The average percent marks obtained by the children of group $A$ is $75$ and the average per cent marks of all the $16$ children is $76 .$ What is the average per cent marks of children of group $B$ ?

Find the average of all the numbers between $6$ and $34$ which are divisible by $5$

The average weight of $21$ boys was recorded as $64 \,kg.$ If the weight of the teacher was added, the average increased by one $kg.$ What was the teacher's weight?