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

For a given frequency distribution,$n=200$,$\Sigma f_{i} d_{i}=0$ and $A=25$. Then,$\bar{x}=\ldots \ldots \ldots \ldots$

Thirty women were examined in a hospital by a doctor and the number of heartbeats per minute were recorded and summarized as follows. Find the mean heartbeats per minute for these women,choosing a suitable method.

Number of heartbeats per minute	$65$-$68$	$68$-$71$	$71$-$74$	$74$-$77$	$77$-$80$	$80$-$83$	$83$-$86$
Number of women	$2$	$4$	$3$	$8$	$7$	$4$	$2$

The mean of ungrouped data and the mean calculated when the same data is grouped are always the same. Do you agree with this statement? Give reason for your answer.

In the formula $\bar{x} = a + \frac{\sum f_{i} d_{i}}{\sum f_{i}}$ for finding the mean of grouped data,$d_{i}$ are deviations from $a$ of:

If $x_{i}$ are the midpoints of the class intervals of grouped data,$f_{i}$ are the corresponding frequencies,and $\bar{x}$ is the mean,then $\sum (f_{i} x_{i} - f_{i} \bar{x})$ is equal to