It is currently 23 Feb 2018, 02:40

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# The formula for the surface of a sphere is S=4πr^2. Two lead balls

Author Message
TAGS:

### Hide Tags

Intern
Joined: 26 Jul 2015
Posts: 20
Location: Chile
GMAT 1: 620 Q38 V37
GMAT 2: 690 Q49 V35
WE: Analyst (Consulting)
The formula for the surface of a sphere is S=4πr^2. Two lead balls [#permalink]

### Show Tags

24 Oct 2015, 11:52
1
KUDOS
2
This post was
BOOKMARKED
00:00

Difficulty:

5% (low)

Question Stats:

83% (01:07) correct 17% (01:09) wrong based on 137 sessions

### HideShow timer Statistics

The formula for the surface of a sphere is $$S=4πr^2$$. Two lead balls have radii 3 inches and 6 inches, respectively. A third ball has surface area equal to the sum of the surface areas of the two smaller balls. What is the radius, in inches, of the third ball?

(A) $$3\sqrt{10}$$
(B) $$3\sqrt{5}$$
(C) $$36π$$
(D) $$45π$$
(E) $$180π$$

It's not really a hard question, but I got it wrong because I didn't take enough time to read carefully the question. So i'll just leave it here for others to avoid my mistake.
Good luck ya'll

PS: Difficulty is probably under 600, but couldn't edit the tags
[Reveal] Spoiler: OA

_________________

Help me with kudos to unlock tests!

Intern
Joined: 23 Aug 2015
Posts: 1
Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls [#permalink]

### Show Tags

24 Oct 2015, 12:28
2
KUDOS
petocities
The surface area of the bigger sphere is = 4π (3^2 +6^2) = 4πr^2
4π (9 + 36) = 4πr^2
45 = r^2
r = \sqrt{45}
= 3 \sqrt{5}
Intern
Joined: 26 Jul 2015
Posts: 20
Location: Chile
GMAT 1: 620 Q38 V37
GMAT 2: 690 Q49 V35
WE: Analyst (Consulting)
Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls [#permalink]

### Show Tags

25 Oct 2015, 09:45
erratic15 wrote:
petocities
The surface area of the bigger sphere is = 4π (3^2 +6^2) = 4πr^2
4π (9 + 36) = 4πr^2
45 = r^2
r = \sqrt{45}
= 3 \sqrt{5}

Yeah, upon revising my calculations I got it right, but on my first time I stopped at 4π(9+36) = 180π. Since that's (E), I just selected it. That's the silly mistake that one has to avoid!
_________________

Help me with kudos to unlock tests!

Intern
Joined: 09 Jan 2017
Posts: 3
Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls [#permalink]

### Show Tags

27 Feb 2017, 10:51
Can please somebody explain how the translation from /sqrt(45) --> 3*/sqrt(5) works?
I got this one right but only because I canceled the 3 obvious wrong ones and then estimated what value r approximately has to be
Math Expert
Joined: 02 Sep 2009
Posts: 43891
Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls [#permalink]

### Show Tags

03 Mar 2017, 01:04
rasti12 wrote:
Can please somebody explain how the translation from /sqrt(45) --> 3*/sqrt(5) works?
I got this one right but only because I canceled the 3 obvious wrong ones and then estimated what value r approximately has to be

$$\sqrt{45}=\sqrt{9*5}=3\sqrt{5}$$

Hope it's clear.
_________________
Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls   [#permalink] 03 Mar 2017, 01:04
Display posts from previous: Sort by