Adam and Hunter each had a collection of seashells. The ratio of Adam's seashells to Hunter's seashells was 2:1. When Hunter found 15 more seashells, the ratio became 1:2. How many seashells did Adam have?

Let Hunter's seashells be H

Then Adam has 2H

When Hunter finds 15 more we have that he now has twice as many as Adam....so....

H + 15 = 2 ( 2H) simplify

H + 15 = 4H

15 = 4H -H

15 = 3H

15 / 3 = H = 5

Adam had 2H = 2 (5) = 10 shells