$A$ student was asked to find the arithmetic mean of the numbers $3, 11, 7, 9, 15, 13, 8, 19, 17, 21, 14$ and $x$. He found the mean to be $12$. What should be the number in place of $x$?

$A, B, C$ and $D$ are four consecutive odd numbers and their average is $42$. What is the product of $B$ and $D$?

Find the average of all prime numbers between $60$ and $90$.

The average of four consecutive odd numbers is $40$. What is the largest number?

The mean of $100$ items was $46$. Later on,it was discovered that an item $16$ was misread as $61$ and another item $43$ was misread as $34$. It was also found that the number of items was $90$ and not $100$. What is the correct mean?

The sum of three consecutive even numbers is $28$ more than the average of these three numbers. Then the smallest of these three numbers is