In a frequency distribution,the mid value of a class is $10$ and the width of the class is $6$. The lower limit of the class is:

State whether each of the following statements is true or false:
$(1)$ The mean of the first four odd natural numbers is $4$.
$(2)$ The highest value of the data is called a range.
$(3)$ The mean of $30$ observations is $30$,then the sum of those observations is $60$.

The class mark of a frequency distribution with equal sizes of two successive classes is $72$ and $77$. Then,the width of the class is ...........

For drawing a frequency polygon of a continuous frequency distribution,we plot the points whose ordinates are the frequencies of the respective classes and abscissae are respectively:

Ten observations $6, 14, 15, 17, x+1, 2x-13, 30, 32, 34, 43$ are written in an ascending order. The median of the data is $24$. Find the value of $x$.

The observations $7, 15, x-1, x+1, 24, 28$ have been arranged in ascending order. If the median of the data is $21$,find the value of $x$.