If the mean of $a, b, c$ is $M$ and $ab + bc + ca = 0$,then the mean of $a^2, b^2, c^2$ is

The average marks in English subject of a class of $24$ students is $56$. If the marks of three students were misread as $44, 45$ and $61$ instead of the actual marks $48, 59$ and $67$ respectively,then what would be the correct average?

Five years ago,the average age of a husband and wife was $23 \text{ years}$. Today,the average age of the husband,wife,and child is $20 \text{ years}$. How old is the child in years?

$A$ batsman has an average of $30$ runs in his $42$ innings. The difference between his maximum and minimum score is $100$. If these two innings are removed,his average for $40$ innings comes down to $28$. What is his minimum score?

Of the three numbers,the average of the first and the second is greater than the average of the second and the third by $15$. What is the difference between the first and the third of the three numbers?

The average age of a family of $6$ members $4$ years ago was $25$ years. Meanwhile,a child was born in this family,and the average age of the whole family is still the same today. The present age of the child in years is?