Which number is missing?

Two natural gas company service personnel, a senior training supervisor and a young trainee, were out checking meters in a suburban neighborhood. They parked their truck at the end of an alley and worked their way to the other end.

At the last house, a woman looking out her kitchen window watched the two men as they checked her gas meter. When they finished, the senior supervisor, proud of his physical condition, challenged his younger co-worker to a foot race back to their truck.

As they approached the truck, they realized that the woman from the last house they checked was huffing and puffing right behind them. They stopped and asked her what was wrong.

Gasping for breath, she replied, "When I saw two gas men running as hard as you two were, I figured I'd better run, too!"