The mean of the following distribution is 18 find the frequency f of c

1.	The mean of the following distribution is 18 find the frequency f of class 19 to 21
Answer»		Class interval	Frequency f\t\t\t11-133123613-156148415-1791614417-19131823419-21f2020f21-2352211023-2542496\xa0{tex}\\sum f _ { i } = 40 + f{/tex}\xa0{tex}\\sum f _ { i } x _ { i } = 704 + 20 f{/tex}\tlet the missing frequency is \'f\'.we know that ,\xa0{tex}Mean= \\frac { \\sum f _ { i } x _ { i } } { \\sum f _ { i } }{/tex}{tex}\\Rightarrow 18 = \\frac { 704 + 20 f } { 40 + f }{/tex}{tex}\\Rightarrow 18(40 + f)=704 + 20f{/tex}{tex}\\Rightarrow 720 + 18f =704 + 20f{/tex}\xa0{tex}\\Rightarrow 2f = 16{/tex}\xa0{tex}\\Rightarrow{/tex}\xa0f = 8

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