Hi rsarashi,

This question can be approached in a couple of different ways. Here's how we can solve it by TESTing THE ANSWERS.

We're told that a circle , with a radius of 4 feet, cut from a uniform piece of sheet metal would weigh 20 pounds. Thus, we can equate area with weight...

A = (pi)(R^2) = (pi)(4^2) = 16pi ft^2 = 20 pounds

Thus, every 16pi ft^2 equates to 20 pounds. We're asked to find the approximate radius of a circle that would weight 60 pounds (which is triple the weight of this circle). Thus, we're looking for an area that is approximately triple this area. Let's TEST Answer B...

Answer B: 7 feet

With a radius of 7 feet, this circle would have an area of...
A = (pi)(R^2) = (pi)(7^2) = 49pi = a little more than triple the area of the other circle. While you can TEST Answers A and C if you like, you'll find that Answer B is closest.

Final Answer: B

GMAT assassins aren't born, they're made,
Rich

rsarashi wrote:A circle with a radius of 4 feet is cut from a piece of sheet metal with uniform thickness. The circle weighs 20 pounds. If another circle is cut from the same sheet and weighs 60 pounds, then its radius is closest to which of the following?


A) 6 feet

B) 7 feet

C) 8 feet

D) 9 feet

D) 10 feet

OAB

Please explain.

Weight of a circular disk = Volume of a circular disk * density of metal = Area of the disk * Thickness of the disk * Density of metal

Since the circular disc is cut from the same metal sheet with a uniform piece, Thickness of the disk and Density of metal are constant

Thus, Weight of a circular disk is proportional to Area of the disk

Weight of a circular disk with 4 feet radius / Weight of a circular disk with x feet radius = Area of the disk with 4 feet radius / Area of the disk with x feet radius

=> 20/60 = π*4^2 / π*x^2
=> 1/3 = 16/x^2
=> x = √48 = ~7 feet

The correct answer: B

Hope this helps!

-Jay
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rsarashi wrote: ↑

Fri Jun 02, 2017 9:12 am

A circle with a radius of 4 feet is cut from a piece of sheet metal with uniform thickness. The circle weighs 20 pounds. If another circle is cut from the same sheet and weighs 60 pounds, then its radius is closest to which of the following?


A) 6 feet

B) 7 feet

C) 8 feet

D) 9 feet

D) 10 feet

OAB

Please explain.

The area of the circle of radius 4 feet is 16π. Let r = the radius of the circle that weighs 60 pounds. Thus, the area of such a circle is πr^2. Since the weight of the circle is proportional to its area, we have:

16π / 20 = πr^2 / 60

Multiplying the equation by 60, we obtain:

48π = πr^2

r^2 = 48

r ≈ 7

Answer: B