The average of $12$ results is $15,$ and the average of the first two is $14.$ What is the average of the rest?

$A$ 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 speed of a cyclist who covers the first,second,and third km at $20, 16,$ and $12 \, km/hr$ respectively (in $km/hr$) is:

$A$ car owner buys petrol at $Rs. 7.50$,$Rs. 8$,and $Rs. 8.50$ per litre for three successive years. What is the approximate average cost per litre of petrol if he spends $Rs. 4000$ each year?

$10$ 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?

If the difference between the average of $x, y$ and $y, z$ is $12,$ then the difference between $x$ and $z$ is