Question

The class mark of the class 90-120 is
(a) 90
(b) 105
(c) 115
(d) 120

Answer 1

Hint: We will define the class limits of a class interval. We will see the definition of the class mark. Then we will see the method to compute the class mark using the upper limit and lower limit of a class.

Complete step by step answer:
The class interval is the range of the values in a class. The endpoints of the class interval are called as class limits. The minimum value in a class is its lower limit and the maximum value in a class is the upper limit. The given class is 90-120. So, the lower limit of this class is 90 and the upper limit is 120.
The class mark is defined as the central value of a class. It is also called as the average of the values of the lower limit and upper limit of the given class. From this definition, we can see the method of computing the class mark as follows,
$\text{class mark = }\dfrac{1}{2}\left[ \text{upper limit + lower limit} \right]$
Substituting the values of the lower limit and the upper limit of the given class, we get
$\text{class mark = }\dfrac{1}{2}\left[ 90+120 \right]$
Simplifying the above expression, we have
$\begin{align}
  & \text{class mark = }\dfrac{1}{2}\times 210 \\
 & =105
\end{align}$
Hence, the class mark of the class 90-120 is 105.

So, the correct answer is “Option B”.

Note: It is important to know the basic definitions associated with a class. The concept of class mark is very essential. We can use the class marks to plot the frequency polygon or frequency curve of a given frequency distribution. Many calculations in statistics use the class mark as a representative of that particular class.