I need detailed help with steps on how to solve, and where each number in the steps comes from , be ready for follow up questions on your answer :

Last year, our return on investment equaled 20%, our residual income equaled $50,000, and our minimum required rate of return (cost of capital) was 12%.

Question : What were our average operating assets (rounded to nearest thousand)?

A) $625,000 B) $250,000 C) $417,000 D) $333,000 E) $575,000

Answer was shown as A) but I need detailed explanations as to how.