If Harriet rolls a number cube 39 times, how many times can she expect to roll a 3 or a 4?

Usually, cubes have 6 sides on them, so the chances of first rolling either 3 or a 4 is 1/3th of a chance. Then, if she rolls it 39 times, I'd do 1/3 of 39, and get 13. She can expect 13 times to roll a 3 or a 4.