4 times 3 is 12, so A is 12 mph. 1/3 divided by 4 is 1/12, so B is 1/12 mph. 1/2 divided by 3 is 1/6, so C is 1/6 mph. Finally, if 6 is 2/3 of the mph, you’d divide 6 by 2 to get 3, then multiply by 3 to get 9, so D is 9 mph. So A would be the fastest.