Your calculation is way off. You forget that the rule of 72 is how long it takes for the money to double. Thus, the final calculation is 2^7.6 which is, as any good computer scientist knows, a little less than 256
Huh? At 5% growth per annum, the economy takes 70/5=14 years to double (I prefer the rule of 70). 112 years since 1900, so that's enough time for the economy to double 112/14=8 times.
The economy grew by a factor of 1.05^112 = 236. Which is close enough to his number to assume that he missed the "doubling" part of the question. 236 is about 7-8 doublings.
Woops, yeah, I thought it asked how many times larger the economy was, not how many times it doubled (which, btw, seems like a weird way to phrase the question to me).
