$A$ class has $20$ boys and $30$ girls. The average age of boys is $11$ years and that of girls is $12$ years. What is the average age of the whole class in years?

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?

The average of marks obtained by $120$ candidates was $35.$ If the average of marks of passed candidates was $39$ and that of failed candidates was $15,$ the number of candidates who passed the examination is

$10 \, kg$ of rice priced at $Rs. \, 12$ per $kg$ is mixed with $6 \, kg$ of rice priced at $Rs. \, 16$ per $kg$. Find the average price in $Rs.$ per $kg$ of the whole mixture?

$A$ man travels by a car to his office at $60 \, km/h$ and returns home along the same route at $20 \, km/h$. Find the average speed of his whole journey (in $km/h$).

The average of $6$ numbers is $3.95$. The average of two of them is $3.4$,while the average of the other two is $3.85$. What is the average of the remaining two numbers?