The following may or may not fit your requirement. But just presenting a possibility.
The minimum requirement is the record needs to be an existing record and the implementation is through actions. So it is possible in detail and summary view such as table view with inline action.
The expressions assumed are [Days]* [Quantity]= [EA] and [EA]/[Quantity]= [Days]
Implementation in detail view is captured in GIFs below
Compute EA action.
Please note the [EA] changes from 700 to 800 when the days are changes from 35 to 40
The [Days] change from 30 to 35 when EA is changed from 600 to 700
The implementation below is in table view with an inline action.
When [EA] is changed from 1500 to 2000 , the [Days] change from 75 to 100
When [Days] are change from 50 to 75, the [EA] changes from 1000 to 1500
