The average age of $30$ girls is $1\, year.$ The average of first $18$ girls is $15\, year.$ Find out the average age of remaining $12$ girls?

Out of four numbers, whose average is $60,$ the first one is one-fourth of the sum of the last three. The first number is

A cricketer had a certain average of runs for his $64$ innings. In his $65^{th}$ innings, he is bowled out for no score on his part. This brings down his average by $2$ runs. His new average of runs is?

The average of $11$ numbers is $10.9$ If the average of the first six is $10.5$ and the average of last six is $11.4,$ find the middle (sixth) number.

There was one mess for $30$ boarders in a certain hostel. On the number of boarders being increased by $10,$ the expenses of the mess were increased by $Rs.\,40$ per month while the average expenditure per head diminished by $Rs.\,2.$ Find out actual monthly expenses. (in $Rs.$)

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 $Rs. \, ?$