The average marks obtained by $150$ students in an examination is $40$. If the average marks of passed students is $60$ and that of the failed students is $20$,what is the number of students who passed the examination?

Out of seven given numbers,the average of the first four numbers is $4$ and that of the last four numbers is also $4$. If the average of all the seven numbers is $3,$ then the fourth number is?

The average weight of a group of $20$ boys was calculated to be $89.4 \text{ kg}$ and it was later discovered that one weight was misread as $78 \text{ kg}$ instead of the correct one of $87 \text{ kg}$. The correct average weight is in $\text{kg}$.

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 $km$):

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

Ten years ago,the average age of $P$ and $Q$ was $20 \text{ years}$. The average age of $P, Q,$ and $R$ is $30 \text{ years}$ now. After $10 \text{ years}$,the age of $R$ will be (in $\text{years}$):