Researchers randomly divided 500 volunteers into two equal-sized groups. One group spent 6 months on the Paleo diet. A control group received a pamphlet about controlling portion sizes. During the study, the Paleo group lost on average 7 pounds (standard deviation of 20 pounds) while the control group lost on average 5 pounds (standard deviation of 12 pounds). Calculate the approximate p-value assessing if the Paleo group achieves greater weight reduction than the control group.