The class length of the excluding type of class $20-30$ in a frequency distribution is .......

Midpoint (Midvalue) of the class $15-30$ is ........

$\Sigma(x_{i} - \bar{x}) = \ldots \ldots \ldots$

In the formula $Z = l + \left( \frac{f_{1} - f_{0}}{2f_{1} - f_{0} - f_{2}} \right) \times c$ for the mode,$l = \ldots \ldots \ldots$

In calculating the mean of grouped data,grouped in classes of equal width,we may use the formula $\bar{x} = a + \frac{\sum f_i d_i}{\sum f_i}$,where $a$ is the assumed mean. $a$ must be one of the mid-points of the classes. Is the last statement correct? Justify your answer.

In the formula $\bar{x} = A + \frac{\Sigma f_{i} u_{i}}{\Sigma f_{i}} \times c$,$c$ represents .......