1000 DS section 17 question 20

Expert replies

killer1387: Legendary Member; Posts: 581; Joined: Sun Apr 03, 2011 7:53 amFollowed by: 5 members; Thanked: 52 times

by killer1387 » Fri Apr 29, 2011 7:43 am

Hey all,

Q. Is 5^k less than 1,000 ?
(1) 5^(k-1)> 3000
(2) 5^(k-1)=(5^k)- 500

I solved the question and found the answer as D. But the correct answer is B. I am confused. Please help me out.

Prashant

Like this post

Source: Beat The GMAT — Data Sufficiency | Fri Apr 29, 2011 7:43 am

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Fri Apr 29, 2011 8:19 am

killer1387 wrote:Hey all,

Q. Is 5^k less than 1,000 ?
(1) 5^(k-1)> 3000
(2) 5^(k-1)=(5^k)- 500

I solved the question and found the answer as D. But the correct answer is B. I am confused. Please help me out.

Prashant

Hi Prashant,

are you sure that you've posted (1) correctly? In its current form, (1) is definitely sufficient.

Algebraically:

5^(k-1)> 3000

multiplying both sides by 5:

5^(k-1) * 5^1 > 15000

5^(k-1+1) > 15000

5^k > 15000

and if 5^k is more than 15000, it's definitely NOT less than 1000.

Also, solving (2):

5^(k-1)=(5^k)- 500

500 = 5^k - 5^(k-1)

500 = 5(5^(k-1)) - 1(5^(k-1))

500 = (5-1)(5^(k-1)

500 = 4((5^(k-1))

125 = 5^(k-1)

5^3 = 5^(k-1)

3 = k-1
4 = k

so 5^k = 625

Is 625<1000? YES.

Since (1) appears to give us a definite "no" and (2) gives us a definite "yes", the question is fundamentally flawed (on the real GMAT you'll never get contradictory answers to the question).

If (1) read:

(1) 5^(k-1)< 3000

then (B) would be correct and the question would be legitimate.

What's the source?

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course

Like this post

manpsingh87: Master | Next Rank: 500 Posts; Posts: 436; Joined: Tue Feb 08, 2011 3:07 amFollowed by: 6 members; Thanked: 72 times

by manpsingh87 » Fri Apr 29, 2011 8:21 am

killer1387 wrote:Hey all,

Q. Is 5^k less than 1,000 ?
(1) 5^(k-1)> 3000
(2) 5^(k-1)=(5^k)- 500

I solved the question and found the answer as D. But the correct answer is B. I am confused. Please help me out.

Prashant

hi i too believe answer should be D ...!!! if the answer is B.. than there might be two possible scenarios, either the given information in the question is not correct, or the answer key is wrong..!!!

O Excellence... my search for you is on... you can be far.. but not beyond my reach!

Like this post

MAAJ: Master | Next Rank: 500 Posts; Posts: 243; Joined: Sun Jul 12, 2009 7:12 amFollowed by: 2 members; Location: Dominican Republic; Thanked: 31 times; GMAT Score:480

by MAAJ » Fri Apr 29, 2011 9:02 am

Answer should be B if 5^k > 1000? (if 5^k < 1000 then D is the answer)

1) 5^k > 600 -> Insufficient
2) 5^k = 625 -> Sufficient

"There's a difference between interest and commitment. When you're interested in doing something, you do it only when circumstance permit. When you're committed to something, you accept no excuses, only results."

Like this post

killer1387: Legendary Member; Posts: 581; Joined: Sun Apr 03, 2011 7:53 amFollowed by: 5 members; Thanked: 52 times

by killer1387 » Fri Apr 29, 2011 11:11 pm

Thanx for the reply,

This question is from a question bank named 1000 DS which a friend of mine suggested for DS practice. This question confused me becoz of the contradictory answers i got from the two statements, in addition, the answer was also different, than what i obtained, in the key.

Prashant.

Like this post

killer1387: Legendary Member; Posts: 581; Joined: Sun Apr 03, 2011 7:53 amFollowed by: 5 members; Thanked: 52 times

by killer1387 » Fri Apr 29, 2011 11:37 pm

Hey,

Q. If a certain grove consists of 36 pecan trees, what was the yield per tree last year?
(1) The yield per tree for the 18 trees in the northern half of the grove was 60 kilograms last year.
(2) The yield per tree for the 18 trees in the eastern half of the grove was 55 kilograms last year.

(Question from 1000DS Section 16 Qusetion 7; Answer as per key: 'E', Doubt: Why 'C' is not correct)

Q. Is the number x between 0.2 and 0.7 ?

(1) 560x < 280
(2) 700x > 280
(Question from 1000DS Section 15 Qusetion 11; Answer as per key: 'C', Doubt: Why 'E' is not correct)

Suggest ur opinions please.

Prashant

Like this post

MAAJ: Master | Next Rank: 500 Posts; Posts: 243; Joined: Sun Jul 12, 2009 7:12 amFollowed by: 2 members; Location: Dominican Republic; Thanked: 31 times; GMAT Score:480

by MAAJ » Sat Apr 30, 2011 9:53 am

Is the number x between 0.2 and 0.7 ?

(1) 560x < 280
x < 280/560
x < 28/56
x < 14/28
x < 1/2
x < 0.5 -> Not Sufficient

(2) 700x > 280
x > 280/700
x > 28/70
x > 14/35
x > 2/5
x > (1/5) * 2
x > 0.2 * 2
x > 0.4 -> Not sufficient

(3) Combined 0.4 < x < 0.5 is sufficient to answer. The answer to the question should be YES (the number x IS between 0.2 and 0.7). Correct answer is (C)

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

559 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review