Ask HN: Quantitative question, any help on shortcuts?

I am currently preparing for a problem solving test and i have a question which seems to be difficult to answer at only 2 minutes given and without a calculator.

So, let's say we have data for a period of 3 years. The number of, for example, apples was 2,100,000 3 years ago and is 500,000 this year. I do have to calculate '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 2 minutes without a calculator? I would be very thankful for any help and i don't have much time left until the actual test.

Thank you!