Example (broken in IE):

flex: 0 1 calc(50% - 20px)

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

Workaround:

flex-basis: calc(50% - 20px)

flex-grow: 0 // Default, can be omitted for this example
flex-shrink: 1 // Default, can be omitted for this example

IE11 ignores calc() functions in the flex shorthand