What is the correct way to work with min/max width?

Question

css

Good evening, please tell me how to do it correctly, if two elements in the parent, then the first child took a width of 400 pixels, if one element, then the first child took the width of the parent?

Now it's like this when there are 2 elements


It should be like this



And if there is only one element then



Here is the code:

Answer 1

&-left {
    width: 100%;
    min-width: 400px;

    background: pink;
    &:last-child {
      width: 100%;
    }
  }