Probability is all about figuring out what you are or not going to get.
One good trick of figuring this out is taking what you don't want, (here its green marbles) take them out, then add all the marbles (G,B,R) and divide. Here you are dividing 8 by 10 which comes out to .80
The only other thing to do is simplify if needed. .80 turns into 8/10 and the only thing that 8 and 10 are divisible by is 2 so it would turn into 4/5
