a) To make things a little simpler consider how much fat there is in 100 fluid ounces of milk. According to the claim, there would be 2 fluid ounces of fat and this is 37% less than the fat in whole milk, so, if there are x fluid ounces of fat in whole milk, 37% less is 0.63x=2 so x=2/0.63=3.175 fluid ounces approx. That means 3.175%.
b) Continuing with the example, 3.175-2=1.175 fluid ounces, which is 1.175%. So the claim is not that great after all!