I would guess and check. For example, let's say there are 15 yellow marbles. Multiply by 5 and add 3 to get 78, which means it's too much. Then I guessed 8. 8 times 5 plus 3 is 43. Then I added 8 to 43 to see how many total, which would make 51. Now it's to less. Then I guessed 10. Having 10 yellow means 53 white, which is 63 total. Too many again. Then I guessed 9. 9 yellow is 48 white, which makes a total of 57. So therefore, the answer is 48 white marbles.