Each U.S. penny weighs 2.5 grams. How many pennies, x, are on a scale if their total weight is 37.5 grams?
(Image: 2.5 plus x equals 37.5; x equals 35)
(Image: 2.5 x equals 37.5; x equals 15)
(Image: x minus 2.5 equals 37.5; x equals 40)
(Image: 2.5 x equals 37.5; x equals 18)