With adaptive layout, how can you find out how much% pixels cost?

Question

css

For example, there is a left (main) content and a right one (and the right one is just a vertical menu).
The thing is, I set a strictly defined width to the right block, for example 55px.
I want the left block to close the free space.
If I would set the right block in%, then it would be possible to do this, the left block - 85%, and the right block - 15%.
But how can you solve this problem if the right one is in pixels?
I mean when resizing, so that the left one decreases relative to the free space, and the left remains as always at 55px.
Here is an example: https://jsfiddle.net/gkLhf35u/

Answer 1

width: calc(100% - 55px);
https://jsfiddle.net/gkLhf35u/1/