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(D) The cumulative frequency of a class is the sum of the frequencies of all classes up to that class.Let $cf_n$ be the cumulative frequency of the $n

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$15$

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(D) The cumulative frequency of a class is the sum of the frequencies of all classes up to that class.Let $cf_n$ be the cumulative frequency of the $n^{th}$ class and $f_n$ be the frequency of the $n^{th}$ class.We know that $cf_n = cf_{n-1} + f_n$.Therefore,$cf_{n-1} = cf_n - f_n$.Given that the cumulative frequency of the fourth class $(cf_4)$ is $25$ and the frequency of the fourth class $(f_4)$ is $10$,we have:$cf_3 = cf_4 - f_4$$cf_3 = 25 - 10 = 15$.Thus,the cumulative frequency of the third class is $15$.

Class	$0-20$	$20-40$	$40-60$	$60-80$	$80-100$
Frequency	$5$	$x$	$20$	$y$	$2$

Class	$10-20$	$20-30$	$30-40$	$40-50$	$50-60$	$60-70$	$70-80$
Frequency	$12$	$30$	$x$	$65$	$y$	$25$	$18$

For a given frequency distribution,the cumulative frequency of the fourth class is $25$ and the frequency of the fourth class is $10$. Then,the cumulative frequency of the third class is ...............

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