Is it possible to write a calculation formula that isn't based on numbers?

I don't have a turnkey solution but remembering my middle school technology lessons with electric resistances, a possible approcah woul be to use 2poweredn computations. Eventually, Xn + Yn would be an unique result that would let you know which combination user selected and so display the appropriate value.

Here is a starting point, see if this could help given that you would need to implement it your way :

function doTheMaths() {
    var severity = {
        "I" : 1, 
        "II" : 2, 
        "III" : 4, 
        "IV" : 8, 
        "N/A" : 16
    };
    
    var probability = {
        "A" : 32,
        "B" : 64,
        "C" : 128,
        "D" : 256,
        "N/A" : 512
        
    }
    
    var results = {
        1:"EH", 
        2:"H", 
        3:"M", 
        4:"L",
    }
    
console.println("I+A"+ (severity.I+probability.A) );
console.println("I+B"+ (severity.I+probability.B ));
console.println("I+C"+ (severity.I+probability.C) );
console.println ("I+C"+ (severity.I+probability.D) );
console.println ("I+n/A"+ (severity.I+probability["N/A"]) );
console.println ("II+A"+ (severity.II+probability.A) );
console.println ("II+B"+ (severity.II+probability.B) );
console.println ("II+C"+ (severity.II+probability.C) );
console.println ("II+C"+ (severity.II+probability.D) );
console.println ("II+n/A"+ (severity.II+probability["N/A"]) );
console.println ("III+A"+ (severity.III+probability.A) );
console.println ("III+B"+ (severity.III+probability.B) );
console.println ("III+C"+ (severity.III+probability.C) );
console.println ("III+C"+ (severity.III+probability.D) );
console.println ("III+n/A"+ (severity.III+probability["N/A"]) );
console.println ("IV+A"+ (severity.IV+probability.A) );
console.println ("IV+B"+ (severity.IV+probability.B) );
console.println ("IV+C"+ (severity.IV+probability.C) );
console.println ("IV+C"+ (severity.IV+probability.D) );
console.println ("IV+n/A"+ (severity.IV+probability["N/A"]) );
console.println ("N/A+A"+ (severity["N/A"]+probability.A) );
console.println ("N/A+B"+ (severity["N/A"]+probability.B) );
console.println ("N/A+C"+ (severity["N/A"]+probability.C) );
console.println ("N/A+C"+ (severity["N/A"]+probability.D) );
console.println ("N/A+n/A"+ (severity["N/A"]+probability["N/A"]) );
}
doTheMaths();

The idea here is that the sum of Xn and Yn is always unique.