In a class, there are $20$ boys whose average age is decreased by $2$ $months,$ when one boy aged $18\, years$ in replaced by a new boy. The age of the new boy is

The average annual income (in $Rs.$) of certain agricultural workers is $S$ and that of other workers is $T$. The number of agricultural workers is $11$ times that of other workers. Then the average annual income (in $Rs.$) of all the workers is

In a class, average height of all students is '$a$ ' cms. Among them, average height of $10$ students is '$b$' cms and the average height of the remaining students is '$c$' cms. Find the number of students in the class. (Here $a>c$ and $b>c$ )

A number, $x,$ equals $80 \%$ of the average of $5,7,$ $14$ and a number $y$. If the average of $x$ and $y$ is $26$ then value of $y$ is

A person travels $\frac{1}{3}$ of his journey by bus at $60\, km / h$ , $\frac{1}{3}$ by scooter at $30\, km / h$ and the rest by walking at $10 \,km / h$. Find his average speed for the whole journey. (in $km/h$)

The average of $25$ results is $20 .$ the average of first $12$ results is $15$ and that of the last $12$ results is $18 .$ Then, the $13^{ th}$ results is