The mean of $10$ observations is $15.7$. If a new observation $19$ is included,the new mean is ..........

The mean of the following data is $55.5$ and the total frequency is $20$. Find the missing frequencies $x$ and $y$.

Class	$30-40$	$40-50$	$50-60$	$60-70$	$70-80$
Frequency	$3$	$x$	$7$	$7$	$y$

The mean of $5$ observations is $16$. If an observation $5$ is replaced by $-5$,the new mean is ...........

The two types of Ogives drawn for a frequency distribution intersect at point $(20, 25)$. Then,the median of the data is .......

Given below is a cumulative frequency distribution showing the marks secured by $50$ students of a class:

Marks	Below $20$	Below $40$	Below $60$	Below $80$	Below $100$
Number of students	$17$	$22$	$29$	$37$	$50$

Form the frequency distribution table for the data.

In the formula $\bar{x} = a + h \left( \frac{\Sigma f_{i} u_{i}}{\Sigma f_{i}} \right)$,for finding the mean of grouped frequency distribution,$u_{i}$ is equal to