Diego bought some raisins and walnuts to make trail mix. Raisins cost $4 a pound and walnuts cost $8 a pound. Diego spent $15 on both ingredients.
Decide if each pair of values could be a combination of raisins and walnuts that Diego bought.

4 pounds of raisins and 2 pounds of walnuts
1 pound of raisins and 1.5 pounds of walnuts
2.25 pounds of raisins and 0.75 pounds of walnuts
3.5 pounds of raisins and 1 pound of walnuts