The age of a tree is determined by the number of concentric circles th

Question

The age of a tree is determined by the number of concentric circles that mark the stem when the trunk of the tree is cut. A tall redwood tree cut whose tree trunk had a 60 feet diameter. If the innermost circle had a diameter that measured 30 inches and the concentric circles had a 15 inch gap between them, how many years old was the tree when it was cut?

A) 23
B) 24
C) 46
D) 47
E) 69

A) 23
B) 24
C) 46
D) 47
E) 69

nick1816 · Answer

Diameter of tree= 60 feet=60*12=720 inches
Age of tree= 720/30=24 years

Sourav700 · Answer

60 FEET equals 720 inches.

After the smallest/ innermost concentric circle, each concentric circle is separated by 15 inches; (720-30)/15= 46 concentric circles after the inner most circle.

Therefore, total number of concentric circles = 46 + 1 = 47

SaiPrasadPatnaik · Answer

720,690,630,........30
720=30+(n-1)30
n= 24

Archit3110 · Answer

we can use AP 
an=a +(n-1)*d
an= 720 inches ; 60*12 ; 
a=30
d=30
solve for n = 24
IMO B

Archit3110 · Answer

SaiPrasadPatnaik 
formula used is wrong 
an=a+(n-1)*d
d is 15 not 30

Archit3110 · Answer

nick1816 
you forgot to use the info given 15 inch gap it seems...

Sourav700 · Answer

Now that I think of it, there are 15 inches on both sides of the circle, so 30 inches could also be correct. Man this is confusing.

Posted from my mobile device

Archit3110 · Answer

well by gap of 15 inches it means from center of 1st circle you can understand like this if first dia is 30 then 2nd circle has dia 30+15 ; 45; 3rd has 30+15+15; 60 and so on ... 
 Sourav700

Sourav700 · Answer

By this logic, 47 is the correct answer,  Archit3110

Posted from my mobile device

nick1816 · Answer

Bro!! It says the gap is 15 cm. This means radius of next circle will increase by 15cm and diameter will increase by 30cm. Hence diameter of second circle is 60cm. Diameter of third circle will be 60+30=90cm and so on. Hence the circle whose radius is 720 is the 720/30=24th.

Look at the diagram, will help you easy to understand.
AB=CD+AD+DB=30+15+15=60cm

Archit3110 · Answer

okay  nick1816  , well my approach and thought was the gap of 15 inch was from center , i took d= 15 my bad ; d should be 30
an=720
d=30
a=30
so
720=30+(n-1)*30
n=24

ScottTargetTestPrep · Answer

We see that 30 inches = 30/12 = 5/2 feet and 15 inches = 15/12 = 5/4 feet. However, if we use the gap of 15-inch as the difference between two consecutive rings, we should use the radius of each ring since the 15-inch gap is the difference between the radii of two consecutive rings. Therefore, the age of the tree (i.e., the number of concentric rings) is:

(30 - 5/4)/(5/4) + 1 = 115/4 x 4/5 + 1 = 23 + 1 = 24

Answer: B

gadde22 · Answer

GMAT actually mentions conversion. Here 1 feet = 12 inches.

So total radius of the trunk = 30 * 12 inches = 360 inches.

Inner most circle has radius = 15 inches.

So from the question, for every 15 inches it will gain 1 year age.

Age of the tree = 360/15 = 24.

Posted from my mobile device

Fdambro294 · Answer

Diameter of the tree trunk in inches is:

60 feet * (12 inches per foot) = 720 inches

Radius of entire tree trunk = (1/2) (Diameter) = 360 inches

The inner most circle has a Diameter = 30 inches

The inner most circle’s radius = 15 inches.

If each gap between concentric circles is 15 inches, then the number of circles will be measured by the Radius of the entire trunk ——-> which is given by how many 15 inch gaps exist between circles AND the 15 inch radius of the inner most circle.

15 * (number of circles) = 360 inches = radius of entire trunk

15 * c = 360

c = 24

Number of circles = 24

Posted from my mobile device

iridescent995 · Answer

The question is completely flawed ...we don't account for the bark of a tree for counting its age, so the last radius should be ignored. The answer should be 23 if we account that, I don't think GMAT will give such a confusing question EVER. https://www.wikihow.com/Determine-the-Age-of-a-Tree

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

The age of a tree is determined by the number of concentric circles th

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

The age of a tree is determined by the number of concentric circles th

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards