David Johnson is a young pharmacist who has been working at a local medium-size pharmacy for three years. He is 30 and has an annual salary of $120,000. David thus far has lived the good life and has almost no savings, but he hopes to start saving around $20,000 per year until he retires. However, he recently received an inheritance of $500,000 from his deceased father. He has no pension plan at work, but says he wants to retire as soon as the kids graduate from college (when he is around 55) and, at that point, have between $4 to $5 million (adjusted for inflation!) What's the average return rate need? Adjust for inflation show work/formula please