Apple Inc. has a largely straightforward business model: It makes hardware, sells it at a premium and pockets the difference. The rapid growth of its App Store is starting to change the formula.

Apple said total billings from paid apps and purchases made within apps for its mobile devices rose 50% last year, from more than $10 billion in 2013. That implies App Store sales of roughly $15 billion, making Apple’s take around $4.5 billion. Apple keeps 30 cents of every dollar that flows through the App Store.

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