There are two groups $A$ and $B$ of a class,consisting of $42$ and $28$ students,respectively. If the average weight of group $A$ is $25 \, kg$ and that of group $B$ is $40 \, kg$,find the average weight of the whole class.

The average age of $15$ students in a class is $15 \text{ years}$. Out of these,the average age of $5$ students is $14 \text{ years}$ and that of the other $9$ students is $16 \text{ years}$. What is the age of the $15^{th}$ student? (in $\text{years}$)

$12 \%$ of Kaushal's monthly salary is equal to $16 \%$ of Nandini's monthly salary. Suresh's monthly salary is half that of Nandini's monthly salary. If Suresh's annual salary is $Rs.\, 1.08$ lakhs,what is Kaushal's monthly salary in $Rs.$?

The average of two numbers is $62$. If $2$ is added to the smaller number,the ratio between the numbers becomes $1:2$. The smaller number is

The average of three numbers is $154$. The first number is twice the second and the second number is twice the third. The first number is

The average marks of $120$ students are $35.$ If the average of passed students was $39$ and failed students was $15,$ then find the number of students who have passed.