What affect do multiple trials have on probability?

Question

If I flip a coin once, the chance of heads is 1/2. If I flip it twice, the chance of heads twice is 1/4. (1/2 times 1/2) But what happens if I flip it twice, but only want heads once. I'd think it was 1/2+1/2, but that would be a probability of 1, which I know it isn't. What is the chance and how do I calculate it?

Answer 1

It is still 1/2!

 

H H

H T

T H

T T