In a research study, the initial number of bi… | GMAT Sprint

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PS · Problem Solving

sub-505

Sequences

In a research study, the initial number of birds was 4. Each month, the number of birds doubled from the previous month's count. What will be the total number of birds after 8 months?

Answer Choices

Correct answer marked below

2^4

4^2

4*(2^8)

Correct

4*(2^7)

4*(8^2)

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Explanation

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Official Explanation

Official

Initially, the number of birds is N = 4. After the first month, the number of birds becomes N1 = 4 x 2 = 8. After the second month, the count is N2 =…

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Comments

andreaJan 9, 2024

thanks for posting

RonAug 24, 2024

thanks for the answer

SarahAug 17

easy one, just remember it doubles each month starting from 4 so you keep multiplying by 2. got C without much trouble

PriteshAug 24

haha I did the exact same thing at first, the wording 'after 8 months' is what saves you

AkashJun 12

wait why is D wrong? I thought doubling 8 times would give 2^7 for some reason

LokeshJun 21

ohh ok that makes sense now, thanks

JayJul 16

this is like a basic geometric sequence where r=2, once you see that it's quick

What this question tests

What it tests

Your ability to spot and extend arithmetic and geometric sequences and work with recursive definitions.

Common trap

Off-by-one errors in indexing (a₀ vs a₁) or confusing an arithmetic difference with a geometric ratio.