How to correctly set css for 2 divs in a row?

Question

css

there are 2 divas

<div style="width:300px;"></div>
<div style="width:100%;"></div>

how to make them on the same line on a large screen, and on a small one, 1 div goes under another?

Answer 1

https://jsfiddle.net/x9uLcgp5/

Answer 2

I advocate flexbox
codepen.io/anon/pen/MKLYvB