The average marks of a group of $20$ students on a test is reduced by $4$ when the topper who scored $90$ marks is replaced by a new student. Find the marks of the new student.

The average of a batsman after $25$ innings was $56$ runs per innings. If after the $26^{th}$ inning his average increased by $2$ runs,then find his score in the $26^{th}$ inning?

Amitabh's average expenditure from $January$ to $June$ is $Rs. 4200$. He spends $Rs. 1200$ in $January$ and $Rs. 1500$ in $July$. What is the average expenditure for the months of $February$ to $July$ (in $Rs.$)?

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

In a competitive examination,the average marks obtained was $45$. It was later discovered that there was some error in computerization and the marks of $90$ candidates had to be changed from $80$ to $50$,and the average came down to $40$ marks. The total number of candidates appeared in the examination is

The average temperature of Monday,Tuesday,Wednesday and Thursday is $60^{\circ}$. The average temperature for Tuesday,Wednesday,Thursday and Friday is $63^{\circ}$. If the ratio of temperature for Monday and Friday is $21: 25,$ then what is the temperature of Friday? (in $^{\circ}$)