Find the class mark of class interval 50-60.

Hint: We will first write the lower limit and upper limit of the class interval. Then, we will substitute the value in the formula, ${\text{class - mark}} = \dfrac{{{\text{Lower limit + upper limit}}}}{{\text{2}}}$ to determine the class-mark of the class-interval.

Complete step-by-step answer:
We have to find the class mark of the class interval 50-60.
In the interval 50-60, the lower limit is 50 and the upper limit is 60.
We know that the class-mark is the average of limits of class-interval.
That is, ${\text{class - mark}} = \dfrac{{{\text{Lower limit + upper limit}}}}{{\text{2}}}$
On substituting the values of lower limit and upper limit, we will get,
$
  {\text{class - mark}} = \dfrac{{{\text{50 + 60}}}}{{\text{2}}} \\
   \Rightarrow {\text{class - mark}} = \dfrac{{110}}{{\text{2}}} \\
   \Rightarrow {\text{class - mark}} = 55 \\
$
Hence, the class mark of the interval 50-60 is 55.

Note: The average of the class limits of an interval is called a class mark or mid-value of the interval. Class marks are also known as the central value of the interval. It helps in calculation of mean and median.