I have a quick Simultaneous Equations question, and it might be very simple, but I just need help getting from one step to the next here.

The question is "If x and y are positive numbers such that x+y=1, which of these could be the value os 100x+200y?"

Then they give a bunch of values. I understand how to test each value once they give me the final combined equation, but I just don't know how they got from one to the other.

It says "Since x+y=1, then y=1-x" that I get. Then they say "y=1-x and 100x+200y can be expressed as 100x+200(1-x)=200-100x"

I need that jump explained please. I feel like it's going to be super simple once I see it, I just need it spelled out. My brain isn't working right now, haha.

THANK YOU TO ANYONE WHO ANSWERS THIS!