Let's assume that after "x" days, the vanilla milkshake machine will have "V" cups left, and the chocolate milkshake machine will have "C" cups left.
The amount of vanilla milkshake left after "x" days can be calculated as:
V = 300 - 15x
The amount of chocolate milkshake left after "x" days can be calculated as:
C = 280 - 20x
To find when the two machines will have the same amount of milkshake left, we need to set V equal to C and solve for x:
300 - 15x = 280 - 20x
5x = 20
x = 4
Therefore, the two machines will have the same amount of milkshake left after 4 days.