Isabel is making face mask. She spends $50 on supplies and plans on selling them for $4 per mask. How many mask does have to make in order to make a profit equal to $90?

Set up the cost function C(m) where m is the number of masks:
C(m) = supply cost
C(m) = 50

Set up the cost function R(m) where m is the number of masks:
R(m) = Sale Price * m
R(m) = 4m

Set up the profit function P(m) where m is the number of masks:
P(m) = R(m) - C(m)
P(m) = 4m - 50

The problems asks for profit of 90, so we set P(m) = 90:
4m - 50 = 90