The average of six numbers is $20 .$ If one number is removed, the average becomes $15 .$ What is the number removed ?

In a class of $100$ students, the mean marks obtained in a subject is $30$ and in another class of $50$ students the mean marks obtained in the same subject is $60 .$ The mean marks obtained by the students of two classes taken together is

The average weight of three boys $A, B$ and $C$ is $54 \frac{1}{3} \,kg,$ while the average weight of three boys $B , D$ and $E$ is $53 \,kg .$ What is the average weight of $A , B , C , D$ and $E$ ?(in $kg$)

$A , B , C$ and $D$ are four consecutive even numbers respectively and their average is $65 .$ What is the product of $A$ and $D ?$

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 weight of $4$ men $A, B, C$ and $D,$ is $67\, kg.$ The $5^{th}$ man $E$ is included and the average weight decreases by $2 \,kg$. A is replaced by $F.$ The weight of $F$ is $4 \,kg$ more than $E$. Average weight deereases because of the replacement of $A$ and now the average weight is $64\, kg.$ Find the weight of $A ?$