Just to clarify for those who search this question up:
The key to this question is that △Y = △C + △S.
To illustrate how we get there:
We know that MPS = △S/△Y, and MPC = △C/△Y and that MPS + MPC = 1.
Combining these two things, that means that △C/△Y + △S/△Y = 1, and if we multiply both sides...