Mad Factory

In a factory, there are 2 identical solid blocks of iron.When the first block is melted and recast into spheres of equal radii r r ,then 14 cubic cm of iron was left. When the second block was melted and recast into spheres each of equal radii 2 r 2r , then 36 cubic cm of iron was left.The volumes of solid blocks and all the spheres are integers. What is the volume of each of the larger spheres o radius 2 r 2r (In cubic cm)?


The answer is 176.

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8 solutions

Parth Chopra
Dec 27, 2013

Let the volume of the sphere with radius r r be equal to a a . Let the volume of the sphere with radius 2 r 2r be equal to 8 a 8a .

Now, we first realize that in order for the spheres of radius r r to have 14 cubic cm of iron left over, a > 14 a > 14 , since no more spheres could have been made from the remaining iron.

Suppose that x x spheres of volume a a and y y spheres of volume 8 a 8a were made. We can say the following:

a x + 14 = 8 a y + 36 ax + 14 = 8ay + 36

Rearranging and factoring, we get:

a ( x 8 y ) = 22 a(x - 8y) = 22

Since the volumes of the spheres are all integers, and a > 14 a > 14 , the only possible way we can satisfy the above equation is by letting a = 22 a = 22 and x 8 y = 1 x - 8y = 1 .

Therefore, the volume of one of the larger spheres, 8 a 8a , is equal to 22 8 22 * 8 or 176 \boxed{176} cubic cm.

I checked the cases using modular arithmetic otherwise my solution is similar

Eddie The Head - 7 years, 3 months ago

i did exactly same way. nicely written.

Piyushkumar Palan - 7 years, 5 months ago

I think you missed the restriction 8 a > 36 8a>36 even though it doesn't alter the result.

Nishant Sharma - 7 years, 5 months ago
Raj Magesh
Dec 27, 2013

Let the volume of each block be V V . Let the volume of each small sphere be w w . Then, each large sphere will have a volume of 8 w 8w , by the laws of similarity. Let the number of small spheres produced be x x and the number of large spheres produced be y y .

V w = x \dfrac{V}{w} = x with a remainder of 14 14 :

V = w x + 14 V = wx + 14 , where w > 14 w>14 since the divisor is always greater than the remainder.

V 8 w = y \dfrac{V}{8w} = y with a remainder of 36 36 :

V = 8 w y + 36 V = 8wy + 36 , where 8 w > 36 8w>36 since the divisor is always greater than the remainder.

From the two above inequalities, we know that w > 14 w>14 .

Rearranging the two equations:

V w x = 14 V-wx = 14

V 8 w y = 36 V - 8wy = 36

Subtracting the first equation from the second,

w x 8 w y = 22 wx - 8wy = 22

w ( x 8 y ) = 22 w(x-8y) = 22

Since, w w is given to be an integer, and x x and y y are by definition integers, w w and x 8 y x-8y must be factors of 22.

The choices we have are:

  1. w = 1 , x 8 y = 22 w = 1, x-8y = 22

  2. w = 2 , x 8 y = 11 w=2, x - 8y = 11

  3. w = 11 , x 8 y = 2 w=11, x-8y = 2

  4. w = 22 , x 8 y = 1 w = 22, x-8y = 1

But we know that w > 14 w>14 . Hence, w = 22 8 w = 8 × 22 = 176 w =22 \Longrightarrow 8w = 8 \times 22 = \boxed{176}

Joel Tan
Dec 27, 2013

Firstly, a sphere of radius 2 r 2r has 2 3 = 8 2^{3}=8 times the volume of the sphere with radius r r .
Let the volume of each block be x x . Then x 14 = r m x-14=rm for some positive integer m m and r > 14 r>14 . The inequality is because the remainder, 14 14 , must be less than the divisor, r r . Similarly, x 36 = 8 r n x-36=8rn for some positive integer n n and 8 r > 36 8r>36 which must hold true if the r = 14 r=14 . Subtracting the second equation from the first, we get ( x 14 ) ( x 36 ) = r m 8 r n (x-14)-(x-36)=rm-8rn Simplifying the left side and factorising the right side gives 22 = r ( m 8 n ) 22=r(m-8n) . This means r r divides 22 as m 8 n m-8n is an integer. So r r can only be 1, 2, 11 or 22. However, since r > 14 r>14 , r = 22 r=22 and thus the volume of the larger sphere is 8 × 22 = 176 8 \times 22=176 .

Sujoy Roy
Dec 27, 2013

Let, volume of one block is x x cubic cm, number of spheres of radii r r is n 1 n_1 and number of spheres of radii 2 r 2r is n 2 n_2 .

Form the given information we get, x = 14 + n 1 × 4 3 π r 3 = 36 + n 2 × 4 3 π ( 2 r ) 3 x=14+n_1\times \frac{4}{3}\pi r^3=36+n_2\times \frac{4}{3}\pi (2r)^3 .

Solving the equations we get, r 3 = 21 4 × ( n 1 8 n 2 ) r^3=\frac{21}{4\times (n_1-8n_2)} .

As 4 3 π r 3 \frac{4}{3}\pi r^3 is integer, we find n 1 8 n 2 22 n_1-8n_2 | 22 . So possible choices of n 1 8 n 2 n_1-8n_2 are 1 , 2 , 11 , 22 1,2,11,22 and for these values possible choices of the volumes for small spheres are 22 , 11 , 2 , 1 22, 11,2,1 respectively and volumes for large spheres are 176 , 88 , 16 , 8 176, 88,16,8 respectively. From these choices 22 22 (for small sphere) and 176 176 (for large sphere) is feasible because for small spheres we are left with 14 14 cubic cm metal and for large spheres we are left with 36 36 cubic cm metal ( 176 > 36 176>36 and 22 > 14 22>14 ).

So answer is 176 \boxed{176}

Let Volume of smaller sphere be 'V'.So volume of larger sphere is '8V'.

Let 'n' be number of smaller spheres and 'k' be number of larger spheres.

nV + 14 = 8kV + 36

V(n - 8k)=22

V=11 n-8k=2

Therefore Volume of larger sphere= 8 *11 = 88

n=18 k=2

Whats wrong with this??

Pratik Vora - 7 years, 5 months ago

how can volume be integer as it formula involves irrational number pi

Daanish bansal - 7 years, 5 months ago
Peren Korkut
Dec 27, 2013

volume of small sphere: v and volume of big sphere: 8v
n.v+14 = m.8v + 36 where m and n are integers
v(n-8m) = 22 since v>14 v = 22 and 8v = 176

Let Volume of smaller sphere be 'V'.So volume of larger sphere is '8V'.

Let 'n' be number of smaller spheres and 'k' be number of larger spheres.

nV + 14 = 8kV + 36

V(n - 8k)=22

V=11 n-8k=2

Therefore Volume of larger sphere= 8 *11 = 88

n=18 k=2

Whats wrong with this??

Pratik Vora - 7 years, 5 months ago

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v should be larger than 14 because 14 cubic cm of iron was left after first cast

Peren Korkut - 7 years, 5 months ago

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yes :p thanks

Pratik Vora - 7 years, 5 months ago
Srikanth Kummara
Jan 24, 2014

A be volume of sphere with radius r therefore 8A is the volume of sphere with 2r let x and y be no: of spheres with rad r and 2r respectively xA+14=8yA+36 (eq1) A=22/(x-8y) since all are integers check for x and y for y=1 x=9 that implies A=22

Satisfies eq 1.

Kim Phú Ngô
Dec 27, 2013

Call x the volume of each of the smaller spheres, of each larger sphere is then 8x.
With m,n,x as integers satisfying x > 14 x>14
we have m x + 14 = n ( 8 x ) + 36 m*x+14=n*(8x)+36 x ( m 8 n ) = 22 = 1 22 = 2 11 x*(m-8n)=22=1*22=2*11
x > 14 x>14 so x = 22 x=22
Answer: 8x = 176 \fbox{176}



William Zhang
Dec 27, 2013

Note: all of the solutions here are wrong; the volume of iron increases as it heats up.

Will you please explain how to solve it in the correct manner by using mathematics and not physics \text{mathematics and not physics} .

Soham Dibyachintan - 7 years, 5 months ago

yes but it eventually cools down... and we make our measurements afterwards.

Shivin Srivastava - 7 years, 5 months ago

Really blunt way to give a solution.............

Rohit Nair - 7 years, 5 months ago

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