"Brazil's inflation rate hit 80 percent per month. At that rate, if eggs cost $1 one day, they'll cost $2 a month later."
Really.
Really.
Divide 72 by the periodic interest rate to find the number of periods to double the money.
Dr. Albert Bartlet of University of Colorado has an excellent lecture on its implications for population growth, environmental degradation, and other big picture problems:
I just worked it out; at 80% inflation it would take 1.18 months to double, not 0.9 like the rule of 72 would suggest. That's a fair difference, and certainly more in line with intuitive expectation.
Not that the OP isn't being overly finicky. Both round to 1, and only one significant digit was given anyway. But still.
For most uses, like prices of eggs, it's close enough.
However, I do keep one non-virtual slide rule around just to whip out and show the kids now and then. Nice mahogony base, teflon slide, the works.
The Planet Money folk unashamedly try and explain economic current affairs and stories as simply as possible. It's essentially their self professed raison d'être and they do a good job of it.
Quibbling about the the accuracy of their simplified examples is missing the trees and the forest.
This serves as a good reminder that you will be swiftly punished on HN for making a comment that doesn't add value.
In my defense, I didn't miss the trees and the forest. I was merely pointing out that the story was dumbed down, as you put it, "as simply as possible." And replying to a post pointing out their overly simplified math. Seriously, implying that 1 x 1.8 = 2 isn't even back-of-the-envelop close.
However I did enjoy learning about how the Real came to be.