Sam is 3 years younger than Marco. Marco is 15 years old. How old is Sam?
Let x represent Sam's age, in years.
(Image: x plus 15 equals 3)
(Image: x plus 3 equals 15)
(Image: 15 divided by x equals 3)
(Image: 3 x equals 15)
(Image: 15 minus x equals 3)