There are two blue marbles so you have a 2/8 probability of drawing a blue marble on each draw since the marble is replaced after the first draw. 2/8 can be simplified to be 1/4 and then converted to be 0.25 per draw. 0.25 plus 0.25 equals 0.5. Since we are dealing with percentages multiply 0.5 x 100 which equals 50. Now take 50 and divided by the total number of marbles in the bag which is 8. 50 divided by 8 equals 6.25. So it would be about 6%.
