# Shortcuts for annual percentage change calculations?

Given the number of apples was 2,100,000 3 years ago, is 500,000 this year and has declined at a constant rate during the stated period. Which is the closest estimate of the annual percentage drop between 3 years ago and this year?

The choices are: (1) 20% (2) 25% (3) 40% (4) 55%

What would be the quickest way to solve it in ~3 minutes without a calculator? I would be very thankful for any help!

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On the exam, you might have thought this through like this. suppose the quantity declines by 50% yearly(you choose 50% because it is easy to work with). then $1 \rightarrow .5 \rightarrow .25 \rightarrow .125$ so a 50% decline is too much. This eliminates choice (4).
Lets try 25%. $1 \rightarrow .75 \rightarrow$ (something greater than .5) $\rightarrow$ (something greater than .375) This tells us that 25% is too low. So the only answer must be 40%