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# Each year for 4 years, a farmer increased the number of trees in a

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e-GMAT Representative
Joined: 04 Jan 2015
Posts: 2943
Re: Each year for 4 years, a farmer increased the number of trees in a  [#permalink]

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04 Nov 2018, 03:28
yashna36 wrote:
Hi,

I tried using geometric progression formula for this ->
x is the no of trees at the end of 1st year, end of 2nd year no of trees is 5x/4. Therefore, common ratio (r) is 5/4

6250=x(r^n-1)
6250= x(5/4)^4-1
6250=x(5/4)^3
x=(6250*64)/125
x=3200

can you please explain to me where has my reasoning gone wrong?

Hey yashna36,
There is nothing wrong in your reasoning - you need to do one more step to get the final answer.

You have assumed x as the number of trees at the end of 1st year, whereas the question is asking us to find the number of trees at the beginning of the 1st year.

Therefore, you need to divide 3200 by 5/4 to get the final answer.

Hope this helps.
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Re: Each year for 4 years, a farmer increased the number of trees in a  [#permalink]

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04 Nov 2018, 12:09
It's a compound interest with rate = 25% , time = 4 year.

6250 = x (1+ 25/100)^4

x = 2560
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Joined: 19 Jan 2018
Posts: 49
Re: Each year for 4 years, a farmer increased the number of trees in a  [#permalink]

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28 Jan 2019, 13:30
imhimanshu wrote:
Each year for 4 years, a farmer increased the number of trees in a certain orchard by 1/4 of the number of trees in the orchard of the preceding year. If all of the trees thrived and there were 6250 trees in the orchard at the end of 4 year period, how many trees were in the orchard at the beginning of the 4 year period.

A. 1250
B. 1563
C. 2250
D. 2560
E. 2752

[/spoiler]

Useful to know your Powers of 4 $$5^4 = 625$$ and $$4^4 = 256$$

"Each year for 4 years, a farmer increased the number of trees in a certain orchard by 1/4 of the number of trees in the orchard of the preceding year"
This translate into:

$$Year 1 = X *$$ $$\frac{5}{4}$$

$$Year 2 = X *$$ $$\frac{5}{4}$$ * $$\frac{5}{4}$$

$$Year 3 = X *$$ $$\frac{5}{4}$$ * $$\frac{5}{4}$$ * $$\frac{5}{4}$$

$$Year 4$$ or 6250 trees $$= X *$$ $$\frac{5}{4}$$ * $$\frac{5}{4}$$ * $$\frac{5}{4}$$ * $$\frac{5}{4}$$
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------

$$6250 = X *$$ $$\frac{5^4}{4^4}$$

$$625 * 10 = X *$$ $$\frac{5^4}{4^4}$$

$$25^2 * 10 = X *$$ $$\frac{5^4}{4^4}$$

$$5^4 * 10 = X *$$ $$\frac{5^4}{4^4}$$ (The $$5^4$$ cancel out, leaving $$4^4$$)

$$4^4 * 10 = X$$

$$256 * 10 = X$$

$$2560 = X$$

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Joined: 22 Jan 2019
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Re: Each year for 4 years, a farmer increased the number of trees in a  [#permalink]

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30 Jan 2019, 14:36
Hi,

I may not be 100% sure, but tell me if you agree: I've solved the problem using the compound rate formula for exercises about loans.

Final value = Initial value * (1+ i)^n, where "i" is the interest rate or growth in this case, and "n" number of years. Therefore:

6250= x*(1+1/4)^4 = x*(5/4)^4 --> x= 5^4*10/(5^4/4^4) =(5^4*10*4^4)/5^4=10*4^4=10*64*4=640*4=2560 --> Answer D
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Posts: 1125
Re: Each year for 4 years, a farmer increased the number of trees in a  [#permalink]

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02 Jul 2019, 04:20
Bunuel wrote:
imhimanshu wrote:
Each year for 4 years, a farmer increased the number of trees in a certain orchard by 1/4 of the number of trees in the orchard of the preceding year. If all of the trees thrived and there were 6250 trees in the orchard at the end of 4 year period, how many trees were in the orchard at the beginning of the 4 year period.

A. 1250
B. 1563
C. 2250
D. 2560
E. 2752

Can someone walk me through the logic behind this question. I am able to solve this by using options as well as by assuming the number of trees = x. However, had the question been, "If all of the trees thrived and there were 6250 trees in the orchard at the end of 15 year period, how many trees were in the orchard at the beginning of the 4 year period". then it would have been difficult to solve.

Thanks

Say the number of trees at the beginning of the 4 year period was x, then:
At the end of the 1st year the number of trees would be $$x+\frac{1}{4}x=\frac{5}{4}*x$$;
At the end of the 2nd year the number of trees would be $$(\frac{5}{4})^2*x$$;
At the end of the 3rd year the number of trees would be $$(\frac{5}{4})^3*x$$;
At the end of the 4th year the number of trees would be $$(\frac{5}{4})^4*x$$;
At the end of the $$n_{th}$$ year the number of trees would be $$(\frac{5}{4})^n*x$$;

So, we have that $$(\frac{5}{4})^4*x=6,250$$ --> $$\frac{5^4}{4^4}*x=5^4*10$$ --> $$x=4^4*10=2,560$$.

If the question were "if all of the trees thrived and there were 6250 trees in the orchard at the end of 15 year period, how many trees were in the orchard at the beginning of the 4 year period", then we would have that: $$(\frac{5}{4})^{15}*x=6,250$$ --> $$x\neq{integer}$$, so it would be a flawed question.

Hope it's clear.

Bunuel
In that time, the maker of the question will replace the figure 6250 with another one so that x has to be integer, right?
Thanks__
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Re: Each year for 4 years, a farmer increased the number of trees in a  [#permalink]

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02 Jul 2019, 04:24
Bunuel wrote:
imhimanshu wrote:
Each year for 4 years, a farmer increased the number of trees in a certain orchard by 1/4 of the number of trees in the orchard of the preceding year. If all of the trees thrived and there were 6250 trees in the orchard at the end of 4 year period, how many trees were in the orchard at the beginning of the 4 year period.

A. 1250
B. 1563
C. 2250
D. 2560
E. 2752

Can someone walk me through the logic behind this question. I am able to solve this by using options as well as by assuming the number of trees = x. However, had the question been, "If all of the trees thrived and there were 6250 trees in the orchard at the end of 15 year period, how many trees were in the orchard at the beginning of the 4 year period". then it would have been difficult to solve.

Thanks

Say the number of trees at the beginning of the 4 year period was x, then:
At the end of the 1st year the number of trees would be $$x+\frac{1}{4}x=\frac{5}{4}*x$$;
At the end of the 2nd year the number of trees would be $$(\frac{5}{4})^2*x$$;
At the end of the 3rd year the number of trees would be $$(\frac{5}{4})^3*x$$;
At the end of the 4th year the number of trees would be $$(\frac{5}{4})^4*x$$;
At the end of the $$n_{th}$$ year the number of trees would be $$(\frac{5}{4})^n*x$$;

So, we have that $$(\frac{5}{4})^4*x=6,250$$ --> $$\frac{5^4}{4^4}*x=5^4*10$$ --> $$x=4^4*10=2,560$$.

If the question were "if all of the trees thrived and there were 6250 trees in the orchard at the end of 15 year period, how many trees were in the orchard at the beginning of the 4 year period", then we would have that: $$(\frac{5}{4})^{15}*x=6,250$$ --> $$x\neq{integer}$$, so it would be a flawed question.

Hope it's clear.

Bunuel
In that time, the maker of the question will replace the figure 6250 with another one so that x has to be integer, right?
Thanks__

Since x represent the number of trees, then it must be an integers, so if you were to replace 6,250 with some other number, you should replace it so that x at the end turns out to be an integer.
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Re: Each year for 4 years, a farmer increased the number of trees in a   [#permalink] 02 Jul 2019, 04:24

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