Grouped table with one missing frequency and the mean given: find the frequency

The same question, asked 2 times: CDS 1 2017*, CDS 2 2019. Every wording below, oldest first.

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Question 1
CDS 1 2017
53 $Consider the following distribution : Class Frequency 0 - 20 17 20 - 40 28 40 - 60 32 60 - 80 f 80 - 100 19 If the mean of the above distribution is 50, what is the value of f ?$
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Question 2
CDS 2 2019
61 $Consider the following grouped frequency distribution : x f 0 - 10 8 10 - 20 12 20 - 30 10 30 - 40 p 40 - 50 9 If the mean of the above data is 25.2, then what is the value of p ?$
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Question 1