In a bag of 20 marbles, 4 are blue. what is the probability that one taken at random will not be blue?

Your answer is 4/5, which is 80% or 0.8

If there are 20 marbles in total in the bag, and 4 blue ones, that means that the probability of taking a blue one is 4/20.

This means that the probability of not taking a blue one is 1 - 4/20, or 20/20 - 4/20, which equals 16/20 = 4/5

I hope this helps! Let me know if you have any questions :)