A cylinder has a base with a circumference of 20pi meters

Question

A cylinder has a base with a circumference of 20pi meters and an equilateral triangle inscribed on the interior side of the base. A marker is dropped into the tank with an equal probability of landing on any point on the base. If the probability of the marker landing inside the triangle is (Sqrt 3)/4 , what is the length of a side of the triangle?

A. 3(sqrt 2pi)
B. 3(sqrt 3pi)
C. 10 pi
D. 10(sqrt 3pi)
E. 20 pi

A. 3(sqrt 2pi)
B. 3(sqrt 3pi)
C. 10*sqrt (pi)
D. 10(sqrt 3pi)
E. 20 pi

Bunuel · Accepted Answer

Since the probability of the marker landing on the portion of the base inside the triangle is \frac{\sqrt{3}}{4} then the portion of the base (circle) inside the triangle must be \frac{\sqrt{3}}{4} of the area of the base. Next: circumference=20\pi=2\pi{r} --> r=10 --> area_{base}=\pi{r^2}=100\pi ; The area of the equilateral triangle is \frac{\sqrt{3}}{4} of the base: area_{equilateral}=\frac{\sqrt{3}}{4}*100\pi --> also the ares of the equilateral triangle is area_{equilateral}=a^2*\frac{\sqrt{3}}{4} , where a is the length of a side --> area_{equilateral}=a^2*\frac{\sqrt{3}}{4}=\frac{\sqrt{3}}{4}*100\pi --> a=10{\sqrt\pi} . Answer: C. Similar question to practice: a-cylindrical-tank-has-a-base-with-a-circumference-of-105453.html Hope it helps.

GyanOne · Answer

Prob of marker landing in the triangle = area of triangle / area of base = sqrt(3)/4

Circumference of base = 20pi meters 
=> Radius of base = 10 meters
=> Area of base = 100pi sq m

Therefore area of the triangle = 100pi * sqrt(3)/4

As the area of an equilateral triangle is sqrt(3)*(side^2)/4, 
any side of the triangle = 10*sqrt(pi)

This doesn't seem to be any of the answer choices.

Joy111 · Answer

sorry missed the sqrt in option C It has now been edited , My bad

A. 3(sqrt 2pi) B. 3(sqrt 3pi) C. 10 sqrt (pi) D. 10(sqrt 3pi) E. 20 pi

paragkan · Answer

Hello All,

See my solution below and please tell me where I am going wrong:
Radius of base = 10 (as derived by you)
Now, if we draw the equilateral triangle inscribed in a circle (as shown in my attachment which is not to scale), then:
OA = radius = 10
and O is the centroid
Centroid divides a median in the ratio 2:1. Hence, OD = OA / 2 = 5

Now, AD is the height of the triangle and in equilateral triangle,
 height = a * sqrt (3) /2
where a = side of triangle

Hence, 15 = a * sqrt (3) /2
Hence a = 10 * sqrt (3)

Where am I going wrong?

smartmanav · Answer

In this question , Triangle is inscribed inside circle so if I am not wrong, centre of the circle will be the circumcentre. As per Maths book of GMAT club relationship between Equilateral triangle and Circcumradius is 
R = a/3^1/2 . We know R = 10 , so why putting the values in this formula getting the right answer ?

venuvm · Answer

In this question , Triangle is inscribed inside circle so if I am not wrong, centre of the circle will be the circumcentre. As per Maths book of GMAT club relationship between Equilateral triangle and Circcumradius is
R = a/3^1/2 . We know R = 10 , so why putting the values in this formula NOT getting the right answer ?

alphabeta1234 · Answer

Hey Bunuel quick question,

If the radius of a circle that inscribed an equaliteral is  r=S\sqrt{3}/3 , where r is the radius and S is the side of the equilateral, should int the answer be  30/\sqrt{3}=S ?

hariprasad · Answer

Could anyone pl clarify my doubt??

For an equilateral triangle circumscribed in a circle,  side = R * root 3  where R is the circumradius.

In this problem, R = 10, So side of the triangle should be  10 * root 3 ..

Pl point where I am going wrong.

holidayhero · Answer

I did the same thing as Alphabeta1234 and Friend29. Can someone explain why this is incorrect? 
 S=\frac{30}{\sqrt{3}}  
 S=10\sqrt{3} .

Thanks

taleesh · Answer

C= 20pi =2pi*10
area of circle= pir^2 = pi10^2=100pi
 Now as question mentions that probability of dropping a marker anywhere on the base is same and in particular in the equilateral triangle is sqrt3/4. So, equilateral triangle's area is sqrt3/4 of total area of circle (100pi) = sqrt3/4 *100pi( for e.g we do 20% of 100).
Now area of equilateral triangle can also be represented = as per its formula = a^2*sqrt3/4
So now when will be equating both the equations 
a^2*sqrt3/4 =sqrt3/4 *100pi 
a^2 = 100 pi
a= sqrt 10pi

earnit · Answer

I cannot seem to understand the highlighted part.

How do we actually relate the given probability to the area of the base?

Bunuel · Answer

P = \frac{(favorable)}{(total)} =\frac{(area \ of \ triangle)}{(area \ of \ circle)} = \frac{\sqrt{3}}{4}  -->  (area \ of \ triangle)=(area \ of \ circle)*\frac{\sqrt{3}}{4} .

Hero8888 · Answer

Why do we need the extra information about probability if finding the radius is enough?

priyatomar · Answer

I am looking at the answer the same way. The side of the triangle can directly be found through the radius of the circle.
Why are we doing it the other way? the answer should be 10 root 3.

goaltop30mba · Answer

:/

if I use the probability in calculating the side of the triangle, i get 10 root pi

If i use 30 60 90 angled side ration method, i am getting 10 root 3

as mentioned by someone above, the radius is sufficient to calculate the side, we actually do not need the probability

experts, please share your views

Saatchii · Answer

If we directly calculate the side of the triangle, using the formula a(3)^1/2, after calculating the radius to be 10, why is the answer coming out to be wrong?

Magoosh   Bunuel   chetan2u  @karishmaveritasprep

chetan2u · Answer

This is a faulty question. The side of the largest equilateral triangle will be  10\sqrt{3} ...
Circumference =2*pi*r=200*pi, so r=10, but we know r= a\sqrt{3}=10.....a=10\sqrt{3} .

But we get the answer or side as  10\sqrt{\pi} , which is greater than 10 \sqrt{3} .
Is it possible to have a greater equilateral triangle than the one which has all three vertices on the circumference.....NO

You can discard the question for two reasons...
1) It gives you a triangle which cannot fit into the base
2) It gives you that triangle is inscribed on the base while it actually is trying to mean that the triangle is contained within the base.

Fdambro294 · Answer

doesn't the question mean to say that the Equilateral Triangle is  PAINTED  inside the Base of the Cylinder?

bumpbot · Answer

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.

A cylinder has a base with a circumference of 20pi meters

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

A cylinder has a base with a circumference of 20pi meters

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