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 of marks obtained by $120$ candidates in a certain examination is $35 .$ If the average marks obtained by passed candidates are $39$ and those of the failed candidates are $15,$ then what is the number of candidates who passed the examination?

Out of $30$ teachers of a school, a teacher of age $60$ $years$ retired. In his place another teacher of age $30$ $years$ was appointed. As a result, the mean age of the teachers will

In a class, average height of all students is '$a$ ' cms. Among them, average height of $10$ students is '$b$' cms and the average height of the remaining students is '$c$' cms. Find the number of students in the class. (Here $a>c$ and $b>c$ )

The average revenue of $13$ consecutive years of a company $Rs.\, 70$ $\,lakhs.$ If the average of first $7 \,years$ is $Rs.\, 65$ $\,lakhs$ and that of last $7\, years$ is $Rs.\, 77$ $\,akhs,$ find the revenue for the $7^{th}$ $year.$ (Rs. in $lakh$)