Suppose that Canada has a total of \$10 billion in \$20 bills in circulation, and each day \$40 million of these \$20 bills passes through one bank or another. A new harder-to-forge version of the \$20 bill is developed, and the banks replace the old bills with new ones whenever they can. How long does it take for the new bills to reach 90% of the total number of $20 bills in circulation?
How long does it take for the new bills to reach 90% of the total number of $20 bills in circulation?
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ordinary-differential-equations