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A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is

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A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 03 Aug 2010, 00:08
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A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is 2 mm thick and its diameter is 15 mm). If the weight of the coin is 30 grams and the volume of aluminum in the alloy equals that of silver, what will be the weight of a coin measuring 1 x 30 mm made of pure aluminum if silver is twice as heavy as aluminum?
36 grams
40 grams
42 grams
48 grams
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 03 Aug 2010, 01:20
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bibha wrote:
A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is 2 mm thick and its diameter is 15 mm). If the weight of the coin is 30 grams and the volume of aluminum in the alloy equals that of silver, what will be the weight of a coin measuring 1 x 30 mm made of pure aluminum if silver is twice as heavy as aluminum?
36 grams
40 grams
42 grams
48 grams


Coin is in the shape of a cylinder.

Coin 1

Volume = pi* \(7.5^2\)*2 = 112.5pi cubic mm

Total coin weight(including both Aluminium and Silver)= 30g but Silver is twice as heavy as aluminium.
So weight of silver = 20g and weight of aluminium =10g

Volume of aluminium = Volume of silver
volume of aluminium = volume of silver = 112.5pi/2 = 56.25pi
So 56.25pi cubic mm of aluminium weighs 10g

Coin 2

Volume = pi* \(15^2\)*1 = 225pi cubic mm
Coin is made of pure aluminium . Also 56.25pi cubic mm of aluminium weighs 10g from above
So total weight of the second coin = (10/56.25pi)*225pi = 40g
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 03 Aug 2010, 03:07
Volume of aluminium = Volume of silver
Would you please explain this step??
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 03 Aug 2010, 19:07
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bibha wrote:
Volume of aluminium = Volume of silver
Would you please explain this step??


See the 2nd statement in the question "volume of aluminum in the alloy equals that of silver". Coin 1 is a mixture of aluminium and silver and in that mixture volume of aluminium is equal to the volume of the silver. So if the total volume of the coin is 112.5pi cubic mm , volume of aluminium in coin1 is 112.5pi/2 cubic mm
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 12 Sep 2011, 09:26
Coin is basically a cylinder.
So volume of coin T= pi r^2 h = pi (7.5)^2 * 2
Coin=Silver+Aluminum
Now total volume of coin(T) = volume of silver + volume of aluminum
Also, volume of silver(Vs)= volume of aluminum(Va)
T= Va+Vb
T=2Va
Va=T/2= pi (7.5)^2 * 2 /2 = pi (7.5)^2

Silver is twice as heavy as aluminum.

Let the weight of aluminum in coin be x
Weight of Silver = 2x
Total weight of coin = 30
x+2x=30
x=10
Weight of Aluminum in coin is 10gm
Wright of Silver in coin is 20gm.

Weight of Aluminum in coin is 10gm and volume is pi (7.5)^2

Now new Aluminum coin is made with dimension 1x30mm.
Volume of this new coin = pi (15)^2*1.

Volume of pi (7.5)^2 contains weight of 10gm of aluminum
Volume of pi (15)^2*1 will contain = 10/ pi(7.5)^ * pi (15)^2 * 1= 40gm

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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 12 Sep 2011, 09:41
Total volume 225*2pi/4
volume of aluminum in the alloy equals that of silver
so volume of only aluminum is half of 225*2pi/4
which is 225*pi/4

Total weight 30 gm
Silver is twice as heavy as aluminium. So
Silver weight in the coin 20 gm and aluminium weight 10 gm

so weight of 225*pi/4 cubic mm of aluminum is 10 gms

Now volume of 1 x 30 mm coin is 225 pi which is 10*4 =40 gms
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 13 Sep 2011, 05:25
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bibha wrote:
A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is 2 mm thick and its diameter is 15 mm). If the weight of the coin is 30 grams and the volume of aluminum in the alloy equals that of silver, what will be the weight of a coin measuring 1 x 30 mm made of pure aluminum if silver is twice as heavy as aluminum?
36 grams
40 grams
42 grams
48 grams


You need to do very few calculations if you understand the relation between the variables.
Let's consider the first coin:
Volume of Silver = Volume of Aluminum
Weight of Silver = 2* Weight of Aluminum
Total weight is 30 gms. Since they are in equal volume but silver is twice as heavy, out of 30 gms, silver must be 20 gms and aluminum must be 10 gms.
(Think what would happen if it was given that volume of aluminum was twice the volume of silver)

Now we need to figure out the relation between the volumes of the two coins.

Volume of first coin \(= pi * (15/2)^2 * 2 = (225/2)*pi\)
Volume of second coin \(= pi * (15)^2 * 1 = (225)*pi\)

Volume of second coin is twice the volume of the first coin. If volume of aluminum in the first coin is V, the volume of the first coin is 2V. The volume of the second coin is 4V. Since it is all Aluminum, volume of Aluminum in the second coin is 4V. Since weight of Aluminum of weight V was 10 gms, weight of Aluminum of volume 4V will be 40 gms.
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 28 Nov 2015, 18:55
ratio of aluminum weight to volume in smaller coin=10 grams/56.25⫪ mm^3
ratio of aluminum weight to volume in larger coin=w grams/225⫪ mm^3
10/56.25=w/225
w=40 grams
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 20 Dec 2015, 18:02
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it's a tough one...
was confused at the beginning with volume/gr...but
since first one has diameter 15, then r=7.5 and h=2.
volume is pi*r^2*h = 7.5^2 * 2 * pi = 102.50
now, volume of silver is equal to volume of aluminium, or each 56.25pi.
we are told that silver is twice as heavy as aluminium. which means that S+A=30, and 2A+A=30, or A=10gr.
now we know that aluminium consists of 56.25pi volume and has 10gr in the mixture.

we need to get a new coin with volume 15^2*1*h, or 225pi.
now, 225pi/56,25pi = 4.
so we have 4 "volumes" of aluminium. 1 volume of aluminium is 10 gr, thus 4 "volumes" is equal to 4*10=40.
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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New post 20 Dec 2015, 21:20
The trick is not to confuse weight and volume. They play separate roles here.
V/2 measures 10 grams.
So,

2V measures 40 grams.
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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is [#permalink]

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Re: A coin made of alloy of aluminum and silver measures 2 x 15 mm (it is   [#permalink] 24 Feb 2018, 07:43
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