Rockjaw spends a total of $80 each week for karate lessons and ballet lessons. If the amount spent on karate lessons increases $10 next week, it will be twice the amount spent on ballet lessons. How much does he spend on ballet lessons?

x - cost of karate lessons

y - cost of ballet lessons

{x + y = 80

{x + 10 = 2y

{x = 80 - y

{80 - y + 10 = 2y

{x = 80 - y

{90 - y = 2y          | + y

{x = 80 - y

{90 = 3y            | : 3

{x = 80 - y

{y = 30

{x = 50

{y = 30

He spends on ballet lesson $30.