six-sided number cube is to be rolled 30 times. How many times do you expect to roll on an even number?
Since each roll of the number cube is independent and has an equal probability of landing on any of the 6 faces, the probability of rolling an even number on any given roll is 1/2 (since there are 3 even numbers and 3 odd numbers).
Therefore, the expected value (or mean) of the number of times an even number will be rolled in 30 rolls is:
Expected value = 30 rolls x (1/2) probability of rolling an even number on any given roll
Expected value = 15
So you would expect to roll an even number 15 times out of the 30 rolls.