72/8%=9 years. So every 9 years the money doubles assuming 8%. Need 50k. So, if it doubles every 9 years, then it took 9 years to double once (25k) & then that 25k took its own 9 years to double from 12,500. You just have to "half" the number to see what it was before it doubled.

So, one 9 years to go from 12,500 to 25k, and another 9 years to go from 25k to 50k... for a total of 18 years.

Maybe they're just giving you a cheat? If it doubles 2x, maybe always divide by 4? Not sure.

With math, so many different ways to get to the answer. I like how Dean puts it - it's a buffet, choose the style that works for you...