Calculate the number 258

Before going to Europe on business, a man drives his Rolls-Royce to a downtown New York City bank and asks for an immediate loan of $5,000. The loan officer, taken aback, requests collateral. "Well then, here are the keys to my Rolls-Royce," the man says. The loan officer promptly has the car driven into the bank's underground parking for safe keeping and gives the man the $5,000. Two weeks later, the man walks through the bank's doors and asks to settle up his loan and get his car back. "That will be $5,000 in principal, and $15.40 in interest," the loan officer says. The man writes out a check and starts to walk away. "Wait, sir," the loan officer says. "You are a millionaire. Why in the world would you need to borrow $5,000?" The man smiles, "Where else could I find a safer place to park my Rolls-Royce in Manhattan for two weeks and pay only $15.40?"