A 10​-sided die is rolled. Find the probability of rolling an even number. The set of equally likely outcomes is shown below. ​{1, 2, 3, 4, 5, 6, 7, 8, 9, 10​}
The probability of rolling an even number on a 10​-sided die is:
There are ten sides on this die. As stated in your question, there are five even numbers and five odd numbers. If we take the amount of even numbers over the total, you get 5/10, which simplifies to 1/2.
