Find the probability of the following event:

tossing a coin heads up or rolling a 3 with a number cube.

I got: 1/2+1/6 = 2/3 but the right answer is 7/12. I don't understand how to get 7/12. Please help.

that will be 1/2 + 1/2 * 1/6 = 1/2 + 1/12 = 7/12

you want P(heads OR (tails and n=3))