Acoin is tossed. what is the theoretical probability of the coin not showing heads?

It would be a 50/50 chance, because there is only two different things it could land on, and for each there is a 50% chance for it to land on that side so.

It's a 50/50 chance, meaning there is a 50% chance for it to be heads and 50% chance for it to be tails. 

Hope this helps!