Two Jobs and Retire Early?

70_hours_week wonders: "A Survey of teachers in a Nevada school district indicate that 40% have a second job. Do you have a second job? Assume you are 30 and since you like to save your money you could semi-retire by age 50. Now, what if you could nearly double your salary working a second job and that meant you could semi-retire at age 40. Would you do it?"