Demography: doubling time: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Demography: doubling time

Sample item

A population grows at a steady annual rate of 2.5%. Using the rule of 70, approximately how many years will it take to double?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. 28 years

Doubling time is approximately 70 divided by the annual percentage growth rate. At 2.5% per year it is 70 / 2.5 = 28 years. The rule is an approximation derived from the exponential growth formula and is useful for projecting population size.

Option by option

  • A. 28 years (correct)

    70 / 2.5 equals 28 years.

  • B. 40 years

    40 years would correspond to a growth rate of 1.75% per year.

  • C. 25 years

    25 years would require a growth rate of 2.8% per year.

  • D. 35 years

    35 years would correspond to a growth rate of 2% per year.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this PSM question

What is the answer to this PSM question?

The best answer is A, 28 years. Doubling time is approximately 70 divided by the annual percentage growth rate.

Why is "40 years" not the answer?

40 years would correspond to a growth rate of 1.75% per year.

Why is "25 years" not the answer?

25 years would require a growth rate of 2.8% per year.

Why is "35 years" not the answer?

35 years would correspond to a growth rate of 2% per year.

What concept does this question test?

It tests Demography: doubling time in PSM. At 2.5% per year it is 70 / 2.5 = 28 years. The rule is an approximation derived from the exponential growth formula and is useful for projecting population size.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Demography: doubling time?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.