The rationale behind the NPV method is straightforward: If a project has NPV = $0, then the project generates exactly enough cash flows (1) to recover the cost of the investment and (2) to enable investors to earn their required rates of return (the opportunity cost of capital). If NPV = $0, then in a financial (but not an accounting) sense, the project breaks even. If the NPV is positive, then more than enough cash flow is generated, and conversely if NPV is negative.
Consider Project L’s cash inflows, which total $150. They are sufficient (1) to return the $100 initial investment, (2) to provide investors with their 10% aggregate opportunity cost of capital, and (3) to still have $18.78 left over on a present value basis. This $18.78 excess PV belongs to the shareholders—the debt-holders’ claims are fixed—so the shareholders’ wealth will be increased by $18.78 if Project L is accepted. Similarly, Yoshikawa’s shareholders gain $19.98 in value if Project S is accepted.
If Projects L and S are independent, then both should be accepted, because both add to shareholders’ wealth, hence to the stock price. If the projects are mutually exclusive, then Project S should be chosen over L, because S adds more to the value of the firm.