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## Q.65 Problem Solving GMAT Review 13th Edition

tagged by: Brent@GMATPrepNow

# This topic has 3 expert replies and 1 member reply

Priyaranjan Junior | Next Rank: 30 Posts
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Target GMAT Score:
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#### Q.65 Problem Solving GMAT Review 13th Edition

Sat Mar 14, 2015 3:32 am

00:00

A

B

C

D

E

## Global Stats

Difficult

The average distance between the Sun and a certain planet is approximately 23*10^14 inches. Which of the following is closest to the average distance between the Sun and the planet, in kilometres? (1 Kilometre is approximately 3.9*10^4 inches)

A) 7.1*10^8
B) 5.9*10^9
C) 1.6*10^10
D) 1.6*10^11
E) 5.9*10^11

Answer is B. As per the OG answer description, it took the approximate value of 23/39 as 2/4 and derived at a value of 5*10^9.But, what will be the answer if the approximate value of 23/39 will be taken as 2/3?

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GMATGuruNY GMAT Instructor
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Sat Mar 14, 2015 4:45 am
The prompt as posted has a typo.

Quote:
The average distance between the Sun and a certain
planet is approximately 2.3 X 10^14 inches. Which of the
following is closest to the average distance between
the Sun and the planet, in kilometers? (1 kilometer is
approximately 3.9 X 10^4 inches.)

(A) 7.1 X 10^8
(B) 5.9 X 10^9
(C) 1.6 X 10^10
(D) 1.6 X 10^11
(E) 5.9 X 10^11

Priyaranjan wrote:
what will be the answer if the approximate value of 23/39 will be taken as 2/3?
(2.3 * 10¹⁴)/(3.9 * 10⁴) ≈ 2/3 * 10¹⁰ ≈ 0.66 * 10¹⁰ = 6.6 * 10⁹.

The closest answer choice is B.

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Brent@GMATPrepNow GMAT Instructor
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Sat Mar 14, 2015 11:10 am
Priyaranjan wrote:
The average distance between the Sun and a certain planet is approximately 23*10^14 inches. Which of the following is closest to the average distance between the Sun and the planet, in kilometres? (1 Kilometre is approximately 3.9*10^4 inches)

A) 7.1 x 10^8
B) 5.9 x 10^9
C) 1.6 x 10^10
D) 1.6 x 10^11
E) 5.9 x 10^11
We can solve the question using equivalent ratios.
We're comparing kilometers and inches, so the ratio is kilometers/inches

1 Kilometre is approximately 3.9 x 10^4 inches
So, we have the ratio: 1/(3.9 x 10^4)

We're told the number of inches to the sun (2.3 X 10^14) and we must find the number of equivalent kilometers.
Let d = the number of kilometers.
So, we have the ratio: 1/(2.3 X 10^14)

We can now write: 1/(3.9 x 10^4) = d/(2.3 X 10^14)
To solve for d, first cross multiply to get: d(3.9 x 10^4) = 1(2.3 X 10^14)
Solve for d to get: d = (2.3 X 10^14)/(3.9 x 10^4)
d = [2.3/3.9][(10^14)/(10^4)]
d = [0.59][10^10]
To get our response into the proper format, rewrite 0.59 as 5.9 x 10^-1 to get...
d = [5.9 x 10^-1][10^10]
d = 5.9 x 10^9

Cheers,
Brent

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misterholmes Senior | Next Rank: 100 Posts
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Mon Mar 16, 2015 1:41 am
Priyaranjan wrote:
The average distance between the Sun and a certain planet is approximately 23*10^14 inches. Which of the following is closest to the average distance between the Sun and the planet, in kilometres? (1 Kilometre is approximately 3.9*10^4 inches)

A) 7.1*10^8
B) 5.9*10^9
C) 1.6*10^10
D) 1.6*10^11
E) 5.9*10^11

Answer is B. As per the OG answer description, it took the approximate value of 23/39 as 2/4 and derived at a value of 5*10^9.But, what will be the answer if the approximate value of 23/39 will be taken as 2/3?
Priyaranjan,

Part 1: It's fine to take the approximation as 2/4, provided you keep in mind that it's less than 23/39. Also fine to estimate at 2/3, keeping in mind that it's more than 23/39. An estimate can be paired with a direction to improve eliminations, so this is a good habit to practice.

Part 2: Assuming the estimate 2/3, the result as 6.6x10^9, and this is greater than the true value.

Regards,
Mike

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Scott@TargetTestPrep GMAT Instructor
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Tue Jun 30, 2015 11:45 am
Priyaranjan wrote:
The average distance between the Sun and a certain planet is approximately 23*10^14 inches. Which of the following is closest to the average distance between the Sun and the planet, in kilometres? (1 Kilometre is approximately 3.9*10^4 inches)

A) 7.1*10^8
B) 5.9*10^9
C) 1.6*10^10
D) 1.6*10^11
E) 5.9*10^11

Answer is B. As per the OG answer description, it took the approximate value of 23/39 as 2/4 and derived at a value of 5*10^9.But, what will be the answer if the approximate value of 23/39 will be taken as 2/3?
Solution:

This problem is a unit conversion with an added twist of scientific notation. We need to convert 2.3 x 10^14 inches to KILOMETERS. We are given that 1 kilometer is approximately 3.9 X 10^4 inches. We also should recognize that we are being asked which of the following is CLOSEST to the average distance between the Sun and the planet, in kilometers. Because we are being asked for an approximation, we can use some estimation here.

To convert 2.3 x 10^14 inches to kilometers, we need to multiply 2.3 x 10^14 inches by the ratio of:

1 km/(3.9 x 10^4 inches)

However, before doing this multiplication, it will make things easier by “cleaning up” each scientific notation expression. Let’s start with 2.3 x 10^14 inches.

2.3 x 10^14 inches

is equivalent to

23 x 10^13 inches

Notice that because we turn 2.3 into 23, or move the decimal one place to the right, we have to then turn 10^14 into 10^13, or move the decimal one place to the LEFT to counterbalance the fact that we’ve moved the decimal one place to the right for 2.3.

Next we can adjust 3.9 x 10^4 inches. However, we can simply round this value up to 4 x 10^4 inches because the question only asks for the closest answer.

Since we’ve rounded 3.9 up to 4, let’s round 23 up to 24 also. That is, we are converting 24 x 10^13 inches into kilometers given that 1 km is approximately 4 x 10^4 inches:

(24 x 10^13 inches) x 1 km/(4 x 10^4 inches)

(24 x 10^13)/(4 x 10^4) km

We can break this work up into two separate calculations:

1) 24/4 = 6

2) 10^13/10^4 = 10^9

We see that the closest answer is 5.9 x 10^9.

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