I was going to buy some scratch cards today when I thought of something puzzling: if I buy two cards, one with a probability of winning money (any amount) of 0.2, and one with 0.4, what's my total odds of winning any money that day?

I'm a bit of a brainlet and forgotten most of my knowledge of statistics. I guess you don't multiply the numbers as that would result in a lower probability. If it was addition of the numbers surely that would mean buying three of the 0.4 cards would guarantee a win.

What's the real answer?