Assuming that the marbles are either green or not green and that the probability of picking a green marble is constant on every trial, we can use probability to make a prediction.
Let p be the probability of picking a non-green marble on one trial, then the probability of picking a green marble is 1 - p.
Since we pick marbles with replacement, each trial is independent of the others, and the probability of picking a non-green marble is the same for every trial.
The number of times we pick a non-green marble in 42 trials follows a binomial distribution with parameters n = 42 and p.
The expected value (or mean) of a binomial distribution is given by np, so in this case, the best prediction for the number of times we will pick a non-green marble is:
42 * p
We don't know the value of p, but if we assume that the probability of picking a non-green marble is 1/2 (i.e., the marbles are evenly split between green and non-green), then the best prediction for the number of times we will pick a non-green marble is:
42 * (1/2) = 21
Therefore, the best prediction possible for the number of times you will pick a marble that is not green is 21.