To perform calculations when specifying a CSS property calc() function can be used. This function takes single expression as
parameter. When writing this expression some rules must be respected:
  -  no empty 
calc()  
  -  there should be an operator between the arguments 
 
  -  there should not be any division by zero 
 
Otherwise calc() function will be invalid and the entire rule using it will be ignored.
Noncompliant code example
.btn {
  border: solid black 1px;
  width: calc(100% 80px);  /* Noncompliant */
}
Compliant solution
.btn {
  border: solid black 1px;
  width: calc(100% - 80px);
}