The average salary of the entire staff in an office is $Rs. 500$ per day. The average salary of officers is $Rs. 750$ and that of non-officers is $Rs. 250$. If the number of officers is $15$,then find the number of non-officers in the office?

$A$ student finds the average of ten $2$-digit numbers. While copying the numbers,by mistake,he writes one number with its digits interchanged. As a result,his answer is $1.8$ less than the correct answer. The difference between the digits of the number in which he made the mistake is:

$A$ cricketer scored some runs in his continuous $9$ innings. He scored $100$ runs in his $10^{th}$ innings and this increased his average by $8$ runs. What was the average of his runs at the end of $10^{th}$ innings?

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

Visitors to a show were charged $Rs. 15$ each on the first day,$Rs. 7.50$ on the second day,$Rs. 2.50$ on the third day,and the total attendance on the three days was in the ratio $2:5:13$ respectively. The average charge per person for the entire three days is (in $Rs.$):

What is the average of the squares of the first $19$ natural numbers?