Already know the correct answer, but can someone explain it

Hi there,

The quiz question is: "Say we have two floated list items with a width of 45% each. Which of the following declarations, if applied to the list items, will perfectly fill the remainder of the parent element’s width?"

The answers can be: "Margin: 10%;" "Margin: 100%;" "Margin: 2.5%;" "Margin: 0%;"

I already know the correct answer is 2,5. But that is only because I remembered it from watching the video. What I would like to know, is if there is someone who can explain it to me, so I can understand why this is the correct answer. I have watch the video over and over agin, but somehow I still don´t understand it, maybe it is because english isn´t my mother language, is there anyone how can explain it to me like I am a six year old kid??