I know what this person was getting at, though probably a bit late for them. Example - if you toss two coins at the same time and you want to know what is the chance that at least one head is tossed, how do you display this in decimal probability. If you add the decimal probability of each toss up (0.5 + 0.5) this does not work as you know it cannot be 100%.
Ans: number of coin tosses = 2, probability of a head being tossed per coin = 0.5
probability of at least one head being tossed = 1 - (0.5/2) = 0.75 (75%)
so addition does not come into it, the equation is x = 1 - (y/z) where x is the overall probability, y is the probability of a single event and z is the number of events.
so if you are trying to add a probability of 0.5 to another probability of 0.5 the equation would be: 1 - (0.5/2) = 0.75
Hope that makes sense.