The average salary of $15$ persons is $Rs. 5,500$. If the salary of one person is added,the average increases to $Rs. 5,700$. What is the salary of this one person in $Rs.$?

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

$10$ years ago,the average age of all $25$ teachers of a girls' college was $45$ years. $4$ years ago,the principal retired at the age of $60$ years. One year later,a new principal aged $54$ years was recruited. Considering the principal as a teacher,what is the present average age of all $25$ teachers?

$A, B, C$ and $D$ are four consecutive odd numbers and their average is $42$. What is the product of $B$ and $D$?

With an average speed of $40 \, km/h$,a train reaches its destination on time. If it goes with an average speed of $35 \, km/h$,it reaches late by $15 \, \text{minutes}$. The total journey is (in $km$):

The mean of the marks obtained by $100$ students is $60$. If the marks obtained by one of the students was incorrectly calculated as $75$ whereas the actual marks obtained by him were $65$,what is the correct mean of the marks obtained by the students?