Let's say I want to interview $123$ students (years $1$ to $6$) and I'll find $40\%$ of them in the first year of the medical school, and $10\%$ of them in the second year, $20\%$ in the third year and so on... (see the table)
Now, I want to keep my sample size ($n = 123$), but consider students only from second year to sixth year ($60\%$ of my original sample). I know the previous $10\%$ of students in the second year will increase, the $20\%$ of student in the third year will increase as well, etc, but I'm struggling to do the basic math to rearrange these fractions.
Can someone just explain me the (basic) step by step I need to perform to solve that?