A bag contains 4 blue, 5 green and 3 red marbles. How many green marbles must be added to the bag so that at least 80 percent of the marbles are green?

[5 + G] /[3+ 4 + 5 + G] =0.8, solve for G

G = 23 Green marble must be added.

Check: [23 + 5] / [3 + 4 + 5 + 23] =28 / 35 =80% Green marbles.