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 , 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 (In cubic cm)?
In a factory, there are 2 identical solid blocks of iron.When the first block is melted and recast into spheres of equal radii
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
I checked the cases using modular arithmetic otherwise my solution is similar
i did exactly same way. nicely written.
I think you missed the restriction 8 a > 3 6 even though it doesn't alter the result.
Let the volume of each block be V . Let the volume of each small sphere be w . Then, each large sphere will have a volume of 8 w , by the laws of similarity. Let the number of small spheres produced be x and the number of large spheres produced be y .
w V = x with a remainder of 1 4 :
V = w x + 1 4 , where w > 1 4 since the divisor is always greater than the remainder.
8 w V = y with a remainder of 3 6 :
V = 8 w y + 3 6 , where 8 w > 3 6 since the divisor is always greater than the remainder.
From the two above inequalities, we know that w > 1 4 .
Rearranging the two equations:
V − w x = 1 4
V − 8 w y = 3 6
Subtracting the first equation from the second,
w x − 8 w y = 2 2
w ( x − 8 y ) = 2 2
Since, w is given to be an integer, and x and y are by definition integers, w and x − 8 y must be factors of 22.
The choices we have are:
w = 1 , x − 8 y = 2 2
w = 2 , x − 8 y = 1 1
w = 1 1 , x − 8 y = 2
w = 2 2 , x − 8 y = 1
But we know that w > 1 4 . Hence, w = 2 2 ⟹ 8 w = 8 × 2 2 = 1 7 6
Firstly, a sphere of radius
2
r
has
2
3
=
8
times the volume of the sphere with radius
r
.
Let the volume of each block be
x
.
Then
x
−
1
4
=
r
m
for some positive integer
m
and
r
>
1
4
. The inequality is because the remainder,
1
4
, must be less than the divisor,
r
.
Similarly,
x
−
3
6
=
8
r
n
for some positive integer
n
and
8
r
>
3
6
which must hold true if the
r
=
1
4
.
Subtracting the second equation from the first, we get
(
x
−
1
4
)
−
(
x
−
3
6
)
=
r
m
−
8
r
n
Simplifying the left side and factorising the right side gives
2
2
=
r
(
m
−
8
n
)
.
This means
r
divides 22 as
m
−
8
n
is an integer. So
r
can only be 1, 2, 11 or 22. However, since
r
>
1
4
,
r
=
2
2
and thus the volume of the larger sphere is
8
×
2
2
=
1
7
6
.
Let, volume of one block is x cubic cm, number of spheres of radii r is n 1 and number of spheres of radii 2 r is n 2 .
Form the given information we get, x = 1 4 + n 1 × 3 4 π r 3 = 3 6 + n 2 × 3 4 π ( 2 r ) 3 .
Solving the equations we get, r 3 = 4 × ( n 1 − 8 n 2 ) 2 1 .
As 3 4 π r 3 is integer, we find n 1 − 8 n 2 ∣ 2 2 . So possible choices of n 1 − 8 n 2 are 1 , 2 , 1 1 , 2 2 and for these values possible choices of the volumes for small spheres are 2 2 , 1 1 , 2 , 1 respectively and volumes for large spheres are 1 7 6 , 8 8 , 1 6 , 8 respectively. From these choices 2 2 (for small sphere) and 1 7 6 (for large sphere) is feasible because for small spheres we are left with 1 4 cubic cm metal and for large spheres we are left with 3 6 cubic cm metal ( 1 7 6 > 3 6 and 2 2 > 1 4 ).
So answer is 1 7 6
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??
how can volume be integer as it formula involves irrational number pi
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??
Log in to reply
v should be larger than 14 because 14 cubic cm of iron was left after first cast
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.
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
>
1
4
we have
m
∗
x
+
1
4
=
n
∗
(
8
x
)
+
3
6
x
∗
(
m
−
8
n
)
=
2
2
=
1
∗
2
2
=
2
∗
1
1
x
>
1
4
so
x
=
2
2
Answer: 8x =
1
7
6
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 .
yes but it eventually cools down... and we make our measurements afterwards.
Really blunt way to give a solution.............
Problem Loading...
Note Loading...
Set Loading...
Let the volume of the sphere with radius r be equal to a . Let the volume of the sphere with radius 2 r be equal to 8 a .
Now, we first realize that in order for the spheres of radius r to have 14 cubic cm of iron left over, a > 1 4 , since no more spheres could have been made from the remaining iron.
Suppose that x spheres of volume a and y spheres of volume 8 a were made. We can say the following:
a x + 1 4 = 8 a y + 3 6
Rearranging and factoring, we get:
a ( x − 8 y ) = 2 2
Since the volumes of the spheres are all integers, and a > 1 4 , the only possible way we can satisfy the above equation is by letting a = 2 2 and x − 8 y = 1 .
Therefore, the volume of one of the larger spheres, 8 a , is equal to 2 2 ∗ 8 or 1 7 6 cubic cm.