I think you can figure out the rest from here. I probably gave you too much help
Another way to look at it: 1/2 of 66 is 33 so if they had exactlly the same number, each would have 33. Also 1/2 of 6 is 3. If Joe now gave 3 of his 33 marbles to Sam, he would have 33- 3= 30 marbles and Sam would have 33+ 3= 36 marbles, so that Sam would have 6 more than Joe.