The average salary of $15$ persons is $Rs.\, 5,500.$ If the salary of one person is added, the average increase to $Rs. \,5,700 .$ What is the salary of this one person in $Rs. \, ?$

The average of five numbers is $27 .$ If one number is excluded, the average becomes $25 .$ The excluded number is

$30$ pens and $75$ pencil altogether were purchased for $Rs.\, 510.$ If the average price of $75$ pencil was $Rs.\, 2$ , then what was the average price of a pen? (in $Rs.$)

Ashish has earned an average of $4200$ dollars for the first eleven months of the year. If he justifies his staying on in the US on the basis of his ability to earn at least $5000$ dollars per $month$ for the entire $year,$ how much should heearn (in $dollars$) in the last $month$ to achieve his required average for the whole $year$ ?

Fifteen movie theatres average $600$ customers per theatre per day. If $six$ of the theaters close down but the total theater attendance stays the same, then the average daily attendance per theatre among the remaining theaters is

The average of first three numbers is double of the fourth number. If the average of all the four numbers is $12,$ find the $4^{th}$ number.