Let x be the number of pounds of pecans. Since the number of pounds of walnuts is 3 times the number of pecans, the number of pounds of walnuts is 3x. The total cost equation is 3.75(3x) + 4.50(x) = 15.75. Solving for x, we get x = 2. Therefore, there are 2 pounds of pecans and 6 pounds of walnuts.
