The average salary of all the staff in an office of a corporate house is $Rs.\, 5,000$. The average salary of the officers is $Rs.\,14,000$ and that of the rest is $Rs.\,4,000.$ If the total number of staff is $500,$ the number of officers ?

A person covers $9\, km$ with a speed of $3 \,km / h , 25 \,km$ with a speed of $5\, km / h$ and $30\, km$ with a speed of $10\, km / h$. Find out the average speed of person in $km / h$ ?

A man spends $Rs.\,1800$ monthly on an average for the first four months and $Rs.\,2000$ monthly for the next eight months and saves $Rs.\,5600$ a $year$. His average monthly income is (in $Rs.$)

The average of first $25$ numbers is $10$ and the average of the next $25$ numbers is $12 .$ The average of all $50$ numbers is

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

The average age of four brothers is $12 \,year.$ If the age of their mother be also included, the average is increased by $5\, year.$ The age of the mother (in $years$) is