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# Of the test animals in a laboratory, 60% are brown rats and 40% are

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Of the test animals in a laboratory, 60% are brown rats and 40% are  [#permalink]

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29 Aug 2017, 23:52
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5% (low)

Question Stats:

91% (01:37) correct 9% (00:42) wrong based on 72 sessions

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Of the test animals in a laboratory, 60% are brown rats and 40% are black rats. If the average brown rat weighs 9.2 ounces and the average black rat weighs 7.7 ounces, what is the average weight of a rat in the laboratory?

A. 7.7 ounces
B. 8.1 ounces
C. 8.45 ounces
D. 8.6 ounces
E. 9.2 ounces

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Re: Of the test animals in a laboratory, 60% are brown rats and 40% are  [#permalink]

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30 Aug 2017, 00:09
Bunuel wrote:
Of the test animals in a laboratory, 60% are brown rats and 40% are black rats. If the average brown rat weighs 9.2 ounces and the average black rat weighs 7.7 ounces, what is the average weight of a rat in the laboratory?

A. 7.7 ounces
B. 8.1 ounces
C. 8.45 ounces
D. 8.6 ounces
E. 9.2 ounces

Suppose their are 10 rats
(9.2*6 + 7.7*4)/10 = (55.2+30.8 = 86)/10 = 8.6
D
9.2-7.7 = 1.5 divided in 6:4 ratio would be 0.9 and 0.6
9.2-0.6 = 8.6
D
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Re: Of the test animals in a laboratory, 60% are brown rats and 40% are  [#permalink]

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30 Aug 2017, 00:12
0.6*9.2+0.4*7.7=5.52+3.08=8.6 so D is correct

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Of the test animals in a laboratory, 60% are brown rats and 40% are  [#permalink]

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30 Aug 2017, 00:30
Bunuel wrote:
Of the test animals in a laboratory, 60% are brown rats and 40% are black rats. If the average brown rat weighs 9.2 ounces and the average black rat weighs 7.7 ounces, what is the average weight of a rat in the laboratory?

A. 7.7 ounces
B. 8.1 ounces
C. 8.45 ounces
D. 8.6 ounces
E. 9.2 ounces

- Let the number of total animal : 100, so brown rats (BR) = 60 and black rats (BL) = 40.
- $$Average weight = \frac{{number of BR}*{BR's average weight}+{number of BL}*{BL's average weight}}{Total Animal} = \frac{(60*9.2)+(40*7.7)}{100} =$$ 8.6 ounces.

D.
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Re: Of the test animals in a laboratory, 60% are brown rats and 40% are  [#permalink]

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31 Aug 2017, 11:03
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Bunuel wrote:
Of the test animals in a laboratory, 60% are brown rats and 40% are black rats. If the average brown rat weighs 9.2 ounces and the average black rat weighs 7.7 ounces, what is the average weight of a rat in the laboratory?

A. 7.7 ounces
B. 8.1 ounces
C. 8.45 ounces
D. 8.6 ounces
E. 9.2 ounces

No need to calculate.

If we have equal number of rates, it means average wight will be in the middle = 8.45. However, there is greater number of brown rats, So the average will tend be slightly higher than 8.45. So it will be 8.6

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Of the test animals in a laboratory, 60% are brown rats and 40% are  [#permalink]

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31 Aug 2017, 11:21
Let the number of rats be x

It has been given the brown rats are 60%(average weight - 9.2)
Also, the black rats are 40%(weight weight - 7.7)

From the questions stem, $$\frac{.6x * 9.2 + .4x * 7.7}{x} =\frac{5.52x + 3.08x}{x} = 8.6$$(Option D)
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Re: Of the test animals in a laboratory, 60% are brown rats and 40% are  [#permalink]

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02 Sep 2017, 07:29
Bunuel wrote:
Of the test animals in a laboratory, 60% are brown rats and 40% are black rats. If the average brown rat weighs 9.2 ounces and the average black rat weighs 7.7 ounces, what is the average weight of a rat in the laboratory?

A. 7.7 ounces
B. 8.1 ounces
C. 8.45 ounces
D. 8.6 ounces
E. 9.2 ounces

We create the following equation for a weighted mean:

0.6(9.2) + 0.4(7.7) = 5.52 + 3.08 = 8.6

Do we have to write questions about test animals in laboratories? Isn’t there a better theme that we can come up with?

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Re: Of the test animals in a laboratory, 60% are brown rats and 40% are   [#permalink] 02 Sep 2017, 07:29
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