Assuming we have 1 red marble, 1 rock, and marbles of 3 different colors, our first step is to add all of those numbers up. 1+1+3=5
Now the probability of getting a red marble depends on how many red marbles there are per a collection of 5 objects, which is the same as saying (# of red marbles)/5
Now as we know, there is 1 red marble so
The probability is 1/5 which is a ratio of 1 to 5, or 1 marble out of every 5 objects, or a 20% chance because 1/5 is 0.2 so we multiply that by 100 and get 20. Hope this helps