The original ratio of red marbles to white marbles is 6:30 and you need to know how many white marbles are expected with 3 red marbles so you have to make the # of red marbles 3 so you can divide by 2 and you'll get 15 white marble or you could've divided both sides by 6 and get the ratio of red marbles to white marbles as 1:5 and you can see that for every one red marble there is 5 white marbles.
