5/6 of the marbles are red or blue. Since half of the marbles are red, you can convert that to sixths which would be 3/6. You can also convert the number of blue marbles into sixths, and 1/3 is equal to 2/6. When you ad the number of blue marbles to the number of red marbles, you get 5/6. I hope this helps you understand the question better.