If Emilia wants the expected number of winners to be 90 out of 200 students, then the probability of winning must be 90/200 = 0.45.
Since the prize wheel is split into 40 equal-sized sections, the probability of winning on any given spin is 1/40 = 0.025.
Therefore, we can set up the equation:
0.025x = 0.45
where x is the number of sections marked as "win".
Solving for x, we get:
x = 0.45/0.025 = 18
So, Emilia should mark 18 of the 40 sections as "win" in order to achieve an expected number of 90 winners out of 200 spins.