math question help, please explain

Question

PLEASE explain and which formula is used sample or population mean?

Answer 1

Neither is used here (sample and population means are used when we collect data about a group for example heights of people).

Mean daily rainfall over four days = 21

So total rainfall over four days = 21 x 4 = 84

You know the rainfall for the first 3 days, so just take that away from the 84 to get the last days rainfall