May you please explain me step by step answer to below question? Please assume that I am a beginner. Please feel free to ask any information needed from me.

Q.) If we want to estimate a 95% confidence interval for the population mean of "TOTAL" marks within 5 marks, what is the sample size required? Please note that you do not know . You may use your present sample of 1000 as a pilot sample.

Note: Population data & sample data are placed in excel file & can be downloaded from this location.

https://docs.google.com/spreadsheets/d/1iyL38A5ipIXT9YLLyFVbE9DIErhUHxcq/edit?usp=sharing&ouid=109601377011834384299&rtpof=true&sd=true

Thanks.