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The question asks for how many years will elapse before the population of the country doubles? In 4 years the population will already be doubled. Shouldn't it be 3 years the correct answer? Please, help me undertand this...

If each year the population of the country grows by 20%, how many years will elapse before the population of the country doubles?

A. 3 B. 4 C. 5 D. 6 E. 7

In 4 years the population, \(X\), of the country will grow to \((1.2)^{4}X = (1.44)^{2}X = 2.0736X\). 3 years is not enough: \((1.2)^3 = 1.728\).

Answer: B

Since after 3 years population is not exactly double but close to it so clearly during the 4th year it would double and at t=4 exact value would be more than double so why isn't 3 the right answer as to be true to the question..3 years have passed and it is during the 4th year that population doubles.

If each year the population of the country grows by 20%, how many years will elapse before the population of the country doubles?

A. 3 B. 4 C. 5 D. 6 E. 7

In 4 years the population, \(X\), of the country will grow to \((1.2)^{4}X = (1.44)^{2}X = 2.0736X\). 3 years is not enough: \((1.2)^3 = 1.728\).

Answer: B

Since after 3 years population is not exactly double but close to it so clearly during the 4th year it would double and at t=4 exact value would be more than double so why isn't 3 the right answer as to be true to the question..3 years have passed and it is during the 4th year that population doubles.

Even though the population doubled during the 4th year, only three years have elapsed. Elapsed literally means what "has passed", and with how the questions is worded, since the fourth year has not passed, IMO A.

If we take the number of population now as 100 then

(1) - 120 (2) - 144 (3) - 172.8 (4) - obviosly 20% of 172.8 is greater than 30 which gives us 2X100 hence in year 4 the population doubles
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I think this is a poor-quality question and I don't agree with the explanation. Hi, Acoording to wording of the problem- "how many years will elapse before the population of the country doubles?"- the answer should be 3 and not 4. So, either change the Answer or change the question wording.

I think this is a high-quality question and I agree with explanation. there is another nice and fast solution that can be applied here : there is rule of thumb that says that in order to find out the number of years needed for an amount to double you divide 70-72 to the growth rate => 70/20=3.5 so 4 years as 3 are not enough

I think this is a high-quality question and I don't agree with the explanation. I think answer should be 3 as stated by others above. Bunuel please help.

Leaving the argument for a about elapsed time aside. A usefull way to solve this question can be by the RULE OF 72- which say at what rate will the any amount or x double .

Took population as 100! for it to double it means it should become 200. 1.now percentage increase from 100 to 200 is 100% 2.now divide 100% by 20% to get 5 years 3.the question asks how many years will elapse before the population of the country doubles? so 5-1= 4 years. I dont know if my approach is correct!