Quote:Better. The easier way to handle these cases is normally to invert the calculation. The chance of them both *failing* to do it is 0.8*0.8 = 0.64. Which gives, as you say, 36% chance of one or the other or both doing it. Your problem is that you originally counted that 4% twice.

 


Exactly. You can't just add probabilities the way you originally did. The coin toss example makes that obvious (and the fact that it's 50-50 makes things even easier...)