The class-mark of the class $130-150$ is:

If the mean of $18, 2x, 27, 12, 3x, 5x$ and $21$ is $24,$ then find $x$.

The observation which is repeated most often in an ungrouped data is called .......... of the data.

The number of books in different shelves of a library are as follows:
$37, 22, 24, 27, 28, 27, 20, 36, 21, 20,$
$29, 30, 29, 36, 30, 25, 27, 32, 24, 28,$
$34, 20, 25, 28, 30, 20, 35, 25, 27, 31$
Represent the data given above by a grouped frequency distribution table,taking the class intervals as $18-22, 22-26, 26-30, 30-34, 34-38$.

The width of each of five continuous classes in a frequency distribution is $5$ and the lower class-limit of the lowest class is $10$. The upper class-limit of the highest class is:

Find the mean and the median of the observations: $72, 15, 28, 31, 32, 51, 44, 43$ and $62$.