A student bought $4$ books for $Rs. \,120$ from one book shop and $6$ books for $Rs.\, 150$ from another. The average price (in rupees), he paid per book was 

The average contribution of $5$ men to a fund is $Rs.\,35.$ A sixth man joins and pays $Rs.\,35$ more than the resultant average of $6$ men. The total contribution of all the six men is (in $Rs.$)

The average of $8$ numbers is $20 .$ The average of first two numbers is $15 \frac{1}{2}$ and that of next three is $21 \frac{1}{3}$. If the sixth number is less than the seventh and eighth numbers by $4$ and $7$ respectively, then the eighth number is

The average of $6$ numbers is $3.95.$ The average of two of them is $3.4,$ while the average of the other two is $3.85 .$ What is the average of the remaining two numbers ?

A person travelled from one place to another at an average speed of $40 \,km / h$ and back to the original place at an average speed of $50\, km / h$. What is his average speed in $km / h$ during the entire trip?

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 $year$?