Since a coin has 2 sides and there is the same probability of getting either side, then each side has a 50% or 0.5 probability. Therefore, in order to calculate the expected value of one coin flip we need to multiply the value of each side by its probability and add those values together like so...
1 * 0.5 = 0.5
0 * 0.5 = 0
Now we add these values together...
0.5 + 0 = 0.5
Finally, we can see that the expected value of one coin flip is 0.5
