MATH PUZZLE: Can you replace...

Four friends spend weeks planning the perfect lake camping and riding trip. 

Two days before the group is to leave Rob's wife puts her foot down and tells him he isn't going. 

Rob's friends are very upset that he can't go, but what can they do. 

Two days later the three get to the camping site only to find Rob sitting there with a tent set up, firewood gathered, and supper cooking on the fire. 

"Dang man, how long you been here and how did you talk your wife into letting you go?" 

"Well, I've been here since yesterday. Yesterday evening I was sitting in my chair and my wife came up behind me and put her hands over my eyes and said 'guess who'?" 

I pulled her hands off and she was wearing a brand new see through nightie. She took my hand and took me to our bedroom. The room had two dozen candles and rose pedals all over. She had on the bed, handcuffs and ropes! She told me to tie and cuff her to the bed and I did. And then she said, "now, you can do what ever you want." 

So here I am.