I was thinking more about how you could show students this problem. I remembered an article in "Teaching Children Math" that used the area model as a visual for percentage problems. I think that same idea might work here. If we used a rectangle to represent Santa's working life, the area of the rectangle is the whole or 100% of his working life. Students could section off the rectangle by the various fractions. The remaining piece is worth 13, we know this from the problem. This will give the students a strong visual in which they can make educated guesses as to the value of the various fractional parts of (percentage of) Santa's working life. You could at this point use the least common denominator as was previously suggested to help convert from the fractions to actual years.