The Stopdecay Company sells an electric toothbrush for $25. Its sales have averaged 8,000 units per month over the past year. Recently, its closest competitor, Decayfighter, reduced the price of its electric toothbrush from $35 to $25. As a result, Stopdecay's sales declined by 1,500 units per month.

What is the arc cross elasticity of demand between Stopdecay's toothbrush and Decayfighter's toothbrush?