# A Very Useful Formula

On the last page, every time we multiplied distance by 10, the time to diffuse increased 100-fold!

10 x as far --> 100 x as much time

What’s going on here? If time to diffuse were directly proportional to
distance, then 10 times the distance would require 10 times as long -- not 100.
This overinflation is like if you went to McDonald’s to buy 10 hamburgers and they charged
you 100 times (not 10 times) the price of a single hamburger. Not fair? Well, at least it's
**not directly proportional **variation.

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