Answer:
The probability of rolling a 3 on a 6-sided cube is 1:6 as there are 6 possibilities and you one want 1 result.
The probability of not rolling 2 or 4 on a 6-sided cube is 2:3 because there are 6 sides and you want to avoid 2 numbers meaning 4 are wanted and then 4:6 can be simplified to 2:3.