Sachin Tendulkar has a certain average for $11$ innings. In the $12^{th}$ innings he scores $120$ runs and thereby increases his average by $5$ runs. His new average is

The mean weight of $150$ students in a class is $60 \, kg$. The mean weight of boys is $70 \, kg$ and that of girls is $55 \, kg$. What is the number of boys in the class?

The average age of the boys in a class is $16 \text{ years}$ and that of the girls is $15 \text{ years}$. The average age for the whole class 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?

The average of $6$ observations is $12$. $A$ new seventh observation is included and the new average is decreased by $1$. The seventh observation is

The sum of the ages of father and son at present is $33 \, \text{years}$. Two years ago, the product of their ages was $28 \, \text{years}$. What is the age of the father and the son (in $\text{years}$)?