I am going to conduct a study in which I'm going to compare the mathematical performance (means) of indigenous students to non-indigenous students in public-urban, private-urban, public-rural and private-rural schools. My problem is that, how to get the sample sizes of each variables (each variables have unequal population), I've seen a similar study to mine in which the sample sizes for each variables are equal (n=20 to be specific, except for the indigenous students where n=18) and I'm not able to find out why 20 and how does the researcher chose 20 as it is not stated. Is it necessary to have an equal sample size for each independent variables? Does n=30 samples for each variables enough? I only have 2 weeks to conduct this one. P.S. I'm not good in this field, can someone help me? Thank you very much

I am going to conduct a study in which I'm going to compare the mathematical performance (means) of indigenous students to non-indigenous students in public-urban, private-urban, public-rural and private-rural schools. My problem is getting the required sample size for each variable (each variable has an unequal population). I've seen a similar study to mine in which the sample sizes for each variable is equal (n=20 to be specific, except for the indigenous students where n=18) and I'm not able to find out why 20 was chosen, as it is not stated. Is it necessary to have an equal sample size for each independent variable? Would n=30 samples for each variable be enough?