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805+ Level|   Geometry|            
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Closed rectangle cardboard outer dimensions= 8*10*12
Closed rectangle cardboard inner dimensions = 7*9*11
Closed cylindrical canister, outer dimension taking base with largest measurement - 9*11 taken
Hence outer diameter of canister = min(9, 11)
--> outer radius of canister =9/2 =4.5 Ans
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is this a 700+ level question?
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I am a little confused about the word "outer".
Does it mean that we should add 0.5+0.5 to the ANS 4.5, from which we obtain 5 as our Final ANS?
Can someone explain it ? thanks a lot!
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BrentGMATPrepNow
parkhydel
The outer dimensions of a closed rectangular cardboard box are 8 centimeters by 10 centimeters by 12 centimeters, and the six sides of the box are uniformly 1/2 centimeter thick. A closed canister in the shape of a right circular cylinder is to be placed inside the box so that it stands upright when the box rests on one of its sides. Of all such canisters that would fit, what is the outer radius, in centimeters, of the canister that occupies the maximum volume?

A. 3.5
B. 4
C. 4.5
D. 5
E. 5.5

PS56710.02

If the OUTER dimensions are 8 cm x 10 cm x 12 cm, and each side is 0.5 cm thick, then the INNER dimensions are 7 cm x 9 cm x 11 cm

Volume of cylinder = pi(radius²)(height)

There are 3 different ways to position the cylinder (with the base on a different side each time).
You can place the flat BASE of the cylinder on the 7x9 side, on the 7x11 side, or on the 9x11 side

If we place the base on the 7x9 side, then the cylinder will have height 11, and the maximum radius of the cylinder will be 3.5 (i.e., diameter of 7).
So, the volume of this cylinder will be (pi)(3.5²)(11), which equals (12.25)(11)(pi), which is a little bit more than 132pi

ASIDE: There's a nice trick for quickly calculating (in your head) the squares of numbers ending with 5 (e.g., 3.5²). See: https://www.gmatprepnow.com/module/gmat ... video/1024

If we place the base on the 7X11 side, then the cylinder will have height 9, and the maximum radius of the cylinder will be 3.5 (i.e., diameter of 7).
So, the volume of this cylinder will be (pi)(3.5²)(9), which equals (12.25)(9)(pi), which is a little bit more than 108pi

If we place the base on the 9x11 side, then the cylinder will have height 7, and the maximum radius of the cylinder will be 4.5 (i.e., diameter of 9).
So, the volume of this cylinder will be (pi)(4.5²)(7), which equals (20.25)(7)(pi), which is a little bit more than 140pi

So, the greatest possible volumeis a little bit more than 140pi, and this occurs when the radius is 4.5


Here's a similar practice question: https://gmatclub.com/forum/the-inside-d ... 28053.html

Answer: C

Cheers,
Brent

Hi BrentGMATPrepNow, what is the outer radius, in centimeters, of the canister mean here as the canister is inside the rectangular cardboard box? Does it imply the radius of the carboard box instead?

If we place the base on the 7x9 side, how do we decide radiuse will be either 7 or 9?

Lastly there are 3 cases, is this due to 7 * 9 *11 ? Thanks Brent
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Kimberly77
BrentGMATPrepNow
parkhydel
The outer dimensions of a closed rectangular cardboard box are 8 centimeters by 10 centimeters by 12 centimeters, and the six sides of the box are uniformly 1/2 centimeter thick. A closed canister in the shape of a right circular cylinder is to be placed inside the box so that it stands upright when the box rests on one of its sides. Of all such canisters that would fit, what is the outer radius, in centimeters, of the canister that occupies the maximum volume?

A. 3.5
B. 4
C. 4.5
D. 5
E. 5.5

PS56710.02

If the OUTER dimensions are 8 cm x 10 cm x 12 cm, and each side is 0.5 cm thick, then the INNER dimensions are 7 cm x 9 cm x 11 cm

Volume of cylinder = pi(radius²)(height)

There are 3 different ways to position the cylinder (with the base on a different side each time).
You can place the flat BASE of the cylinder on the 7x9 side, on the 7x11 side, or on the 9x11 side

If we place the base on the 7x9 side, then the cylinder will have height 11, and the maximum radius of the cylinder will be 3.5 (i.e., diameter of 7).
So, the volume of this cylinder will be (pi)(3.5²)(11), which equals (12.25)(11)(pi), which is a little bit more than 132pi

ASIDE: There's a nice trick for quickly calculating (in your head) the squares of numbers ending with 5 (e.g., 3.5²). See: https://www.gmatprepnow.com/module/gmat ... video/1024

If we place the base on the 7X11 side, then the cylinder will have height 9, and the maximum radius of the cylinder will be 3.5 (i.e., diameter of 7).
So, the volume of this cylinder will be (pi)(3.5²)(9), which equals (12.25)(9)(pi), which is a little bit more than 108pi

If we place the base on the 9x11 side, then the cylinder will have height 7, and the maximum radius of the cylinder will be 4.5 (i.e., diameter of 9).
So, the volume of this cylinder will be (pi)(4.5²)(7), which equals (20.25)(7)(pi), which is a little bit more than 140pi

So, the greatest possible volumeis a little bit more than 140pi, and this occurs when the radius is 4.5


Here's a similar practice question: https://gmatclub.com/forum/the-inside-d ... 28053.html

Answer: C

Cheers,
Brent

Hi BrentGMATPrepNow, (1) what is the outer radius, in centimeters, of the canister mean here as the canister is inside the rectangular cardboard box? Does it imply the radius of the carboard box instead?
Since the authors told us about the inner and outer dimensions of the box, they probably felt it was necessary to note that the measurements of the cylindrical canisters were the outer measurements.

(2) If we place the base on the 7x9 side, how do we decide radius will be either 7 or 9?
If the base is placed on the 7 x 9 side of the box, then the greatest diameter of the cylinder is 7 (otherwise it wouldn't fit into the box).

(3) Lastly there are 3 cases, is this due to 7 * 9 *11 ? Thanks Brent
The three different dimensions of the box it means there are three different cases.
Case 1: The base of each canister is placed on the 7x9 of the box
Case 2: The base of each canister is placed on the 7x11 of the box
Case 3: The base of each canister is placed on the 11x9 of the box
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Great explanation always BrentGMATPrepNow thanks Brent :thumbsup: :please:
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