MATH PUZZLE: Can you replace...

A guy calls a company and orders their 5-day, 10 lb. weight loss program. The next day, there's a knock on the door and there stands before him a voluptuous, athletic, 19 year old babe dressed in nothing but a pair of Nike running shoes and a sign round her neck. She introduces herself as a representative of the weight loss company. The sign reads, "If you can catch me, you can have me."

Without a second thought, he takes off after her. A few miles later, huffing and puffing, he finally catches her and has his way with her. The same girl shows up for the next four days and the same thing happens. On the fifth day, he weighs himself and is delighted to find he has lost 10 lb. as promised. He then calls the company and orders their 5-day, 20 lb. program.
The next day there's a knock at the door and there stands the most stunning, beautiful, sexy woman he has ever seen in his life. She is wearing nothing but Reebok running shoes and a sign around her neck that reads, "If you can catch me you can have me."
Well, he's out the door after her like a shot! This girl is in excellent shape and it takes him quite a while to catch her but when he does, it's definitely worth every muscle cramp and wheeze. So for the next four days the same routine happens. Much to his delight, on the fifth day he weighs himself only to discover that he has lost another 20 lb. as promised.
He decides to go for broke and calls the company to order the 7-day 50 lb. program. "Are you sure?" asks the representative on the phone. "This is our most rigorous program."
"Absolutely," he replies, "I haven't felt this good in years."
The next day there's a knock at the door and when he opens it he finds this huge, muscular, 7ft man standing there wearing nothing but pink running shoes and a sign around his neck that reads, "I'm Francis. If I catch you, you're mine...".