<div class="container">
<div class="block element-1">100% - 50px</div>
<div class="block element-2">50px</div>
</div>

/*Setup Environment */
/*
boring stuff
*/
html {
  box-sizing: border-box;
}

:root {
  background-color: #373150;
  font-size: 16px;
  font-weight: 400;
  text-rendering: geometricPrecision;
  line-height: 1.4;
  text-decoration-skip: ink;
  font-family: -apple-system-headline, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
}

*, *:before, *:after {
  box-sizing: inherit;
}

body {
  margin:0;
  padding:0;
}

.block {
  display:inline-block;
  background-color:#EF6174;
  color:white;
  font-size:0.8em;
  text-align:center; 
  line-height:4em;
}

/*
Calc Demo 100% Fluid Width.
--
The Width of element 1 is determined by the size of the container width, minus the spacing and width of element 2.

Useful in a responsive, re-usable widget scenario. Since the overall size is controlled by 1 container width, but adaptable to fit in any size.

*/

.container {
  display:block;
  max-width:900px;
  margin:0 auto 20px auto;
}

/* calc( 100% width - width of element 2)*/
.element-1 {
  float:left;
  height:50px;
  width: calc( 100% - 50px );
}

/*Notice how calc can also account for the additioanl spacing appled via margin-left here. */
.element-2 {
  float:left;
  width: 40px;
  height:50px;
  margin-left:10px;
}