Priscila's age =k Mary's age =b Mary is 15 years older than Priscila. This means that if you add 15 years to Priscila's age, you will get Mary's age or if you subtract 15 years from Mary's age you will get Priscila's age. Mary's age = Priscila's age +15 b=k+15 Now let 15 be alone at the right hand side of the equation, thus move the k to the left hand side. Its positive ( + ) so it becomes negative (-) b−k=15