How much paint do you need?

Geometry Level 1

You have bought a kind of paint that could paint 50 m 2 \text{m}^{2} per quart. Then you used approximately 6.28 quarts of paint to color the surface of a sphere. Now you wish to paint a cube that has a side length the same as the sphere's radius. How much paint do you need, in quarts?

(P.S. use 3.14 as an approximation for pi., and m \text{m} means meter.)

2 5 3 6

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4 solutions

let A S A_S be the area of the sphere

by ratio and proportion, we get

A S 6.28 = 50 1 \dfrac{A_S}{6.28}=\dfrac{50}{1} \implies A S = 314 m 2 A_S=314~m^2

the area of the sphere is given by the formula A S = 4 π r 2 A_S=4 \pi r^2 , substituting, we get

314 = 4 ( 3.14 ) ( r 2 ) 314=4(3.14)(r^2) \implies r 2 = 25 r^2=25 \implies r = 5 r=5

the surface area of a cube is given by the formula A C = 6 a 2 A_C=6a^2 , but a = r = 5 a=r=5 , substituting, we get

A C = 6 ( 5 2 ) = 6 ( 25 ) = 150 m 2 A_C=6(5^2)=6(25)=150~m^2

let N N be the number of quarts needed for the cube, then

N = 150 m 2 ( 1 q u a r t 50 m 2 ) = N=150~m^2\left(\dfrac{1~quart}{50~m^2}\right)= 3 q u a r t s \color{#D61F06}\large \boxed{3~quarts}

Margaret Zheng
Feb 21, 2016

The sphere's surface area, as implied by the given conditions, is 50 × 6.28 = 314 m 2 50 \times 6.28 = 314 m^{2} . According to the formula for the surface area of a sphere ( S A = 4 π r 2 SA=4πr^{2} ), it is clear that the radius of the sphere is 5 m 5m . Plug in 5 for the surface area formula for cube, and you will get 150 m 2 m^{2} . This number divided by 50 m 2 m^{2} would be 3 \boxed {3} .

It is given in the problem that 1 q u a r t = 50 m 2 1~quart=50~m^2 . So the surface area of the sphere is 6.28 q u a r t s 6.28~quarts converted to m 2 m^2 . Convert:

6.28 q u a r t s ( 50 m 2 1 q u a r t ) = 314 m 2 6.28~quarts\left(\dfrac{50~m^2}{1~quart}\right)=314~m^2

Solve for the radius of the sphere by using the formula: s = 4 π r 2 s=4\pi r^2 where s s is the surface area of the sphere and r r is the radius of the sphere. Substitute:

314 = 4 ( 3.14 ) r 2 314=4(3.14)r^2 \implies r 2 = 25 r^2=25 \implies r = 5 r=5

The formula for the surface area of the cube is 6 a 2 6a^2 where a a is the edge length. Substitute:

A = 6 ( 5 2 ) = 150 m 2 A=6(5^2)=150~m^2

So the number of quarts needed is 150 m 2 ( 1 q u a r t 50 m 2 ) = 3 q u a r t s 150~m^2\left(\dfrac{1~quart}{50~m^2}\right)=\boxed{3~quarts}

The surface area of the sphere is,

S A S_{A} = = 6.28 q u a r t s 6.28 quarts ( 50 m 2 1 q u a r t \frac{50 m^2}{1 quart} ) = = 314 m 2 314m^2

4 p i 4pi * r 2 r^2 = = 314 314

r r = = 5 5

The surface area of the cube is,

S A S_{A} = = 6 5 2 6*5^2 = = 150 m 2 150m^2

150 m 2 150m^2 ( 1 q u a r t 50 m 2 \frac{1 quart}{50m^2} ) = = 3 q u a r t s \boxed{3 quarts}

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