I need help? I need help getting started?

This is what i found “an explicit formula designates the nth term of the sequence, as an expression of n (where n = the term's location). it defines the sequence as a formula in terms of n. this example is an arithmetic sequence(the same number, 5, is added to each term to get to the next term).” i hope this you for what your looking for. can you plz mark me brainliest?

The answer is a 5/20 because if u add all of the marbles there are 20 and there are 5 red marbles so 5/20 is the answer mark as brainliest