Each U.S. penny weighs 2.5 grams. How many pennies (x) are on a scale if their total weight is 37.5 grams

2.5 + x = 37.5
x = 35

2.5 x = 37.5
x = 35

x - 2.5 = 37.5
x = 40

2.5 x = 37.5
x = 18

Please help!