~ ANSWER=1/2 ~
Simple probability is found by counting all the results which fit requirements and dividing by all possible results.
To find probability of two results in a row, multiply chance of first result by chance of second result.
Since you are replacing the marble before the second draw, we don’t have to figure out the various changes in odds for the different possible first draws. It’s just that simple.
There are 5 white marbles
There are 4 red marbles
There are always 20 marbles in all
5/20*4/20=1/4*1/5=1/20 or 1/2
By coincidence, the same as the chance of drawing the white marble in one draw.