Graphical Representation of Data

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:

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

The class mark of the class $90‐120$ is

 The class mark of the class $16‐24$ is _____.