$A$ 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:

The average age of a group of $9$ students is $20\, \text{years}$. When $6$ more students joined the group,the average age falls by $2\, \text{years}$. The average age of the new students is:

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 he earn (in dollars) in the last month to achieve his required average for the whole year?

The average age of Seema,Sapna,Asha,Kavita,and Atrye is $40 \text{ years}$. The average age of Seema and Sapna is $35 \text{ years}$,and the average age of Asha and Kavita is $42 \text{ years}$. What is the age of Atrye?

In a set of prime and composite numbers,the number of composite numbers is twice the number of prime numbers,and the average of all the numbers in the set is $9$. If the number of prime numbers and composite numbers are exchanged,then the average of the set of numbers increases by $2$. If,during the exchange of the numbers,the average of the prime numbers and composite numbers individually remained constant,then what was the ratio of the average of composite numbers to the average of prime numbers (initially)?

The sum of three consecutive odd numbers is $38$ more than the average of these numbers. What is the first of these numbers?