The average age of $5$ sisters is $20 \,year.$ If the age of the youngest sister be $4 \,year,$ what was the average age of the group of sisters at the birth of the youngest sister in $year$ ?

A student was asked to find the arithmetic mean of the numbers $3,11,7,9,15,13,8,19,17,21,14$ and $x$. He found the mean to be $12 .$ What should be the number in place of $x ?$

The average of first $25$ numbers is $10$ and the average of the next $25$ numbers is $12 .$ The average of all $50$ numbers is

The average marks of $40$ students in an examination was $25 .$ It was later found that the marks of one student had been wrongly entered as $73$ instead of $37.$ What is the value of correct average

A man spends on an average $Rs.\, 269.47$ for the first $7 \,months$ and $Rs.\, 281.05$ for the next $5\, months.$ Find out his monthly salary if he saved $Rs.\,308.46$ during the $year.$ (in $Rs.$)

In a competitive examination, the average marks obtained was $45 .$ It was later discovered that there was some error in computerization and the marks of $90$ candidates had to be changed from $80$ to $50$ , and the average came down to $40$ marks. The total number of candidates appeared in the examination is