In a school with $600$ students, the average age of the boys is $12$ $years$ and that of the girls is $11\, years.$ If the average age of the school is $11 \,years$ and $9 \,months,$ then the number of girls in the school is

The average of $50$ numbers is $38 .$ If two numbers, namely, $45$ and $55$ are discarded, what is the average of the remaining numbers ?

The average weight of a class of $24$ students is $35 \,kg$. If the weight of the teacher is included the average rises by $400\, g.$ The weight of the teacher is (in $kg$)

The average of $25$ results is $20 .$ the average of first $12$ results is $15$ and that of the last $12$ results is $18 .$ Then, the $13^{ th}$ results is

The mean of $19$ observations is $4 .$ If one more observation of $24$ is added to the data, the new mean will be

A student finds the average of $10,2$ digit numbers. If the digits of one of the number is interchanged, the average increases by $3.6 .$ The difference between the digits of the $2 -$digit numbers is