The average age of three boys is $15$ $year.$ If the ratio of their ages is $3:5: 7,$ what is the age of the oldest boy in $year$?

The average of prime numbers between $1$ and $20$ is

A professional institute's total expenditure on students for a particular course is partly fixed and partly varies linearly with the number of students. The average expense per student is $Rs.\,615$ when there are $24$ students and $Rs.\,465$ when there are $40$ students. What is the average expense when there are $60$ students ? (in $Rs.$)

The average of $30$ numbers is $15 .$ The average of the first $18$ numbers is $10$ and that of the next $11$ numbers is $20 .$ The last number is

The average salary of all the associates in a team is $Rs.\,16000.$ The average salary of $7$ senior associates is $Rs.\,24000$ and the average salary of the rest is $Rs.\, 12000$. How many associates work in that team?

The average of the boys in a class is $16\, years$ and that of girls is $15\, years.$ The average age for the whole class is