Sets M and N contain exactly m and n elements, respectively. What is the value of n?
(1) 7m=8n
(2) The intersection of M and N contains exactly 0.4m elements.
Combined sets and ratio problem
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 199
- Joined: Sat Apr 26, 2014 10:53 am
- Thanked: 16 times
- Followed by:4 members
- GMAT Score:780
(1) NOT SUFFICIENT. We know m & n must be positive integers, but there are numerous pairs for which this equation holds. m = 8, n = 7; m = 16, n = 14; etc.Mo2men wrote:Sets M and N contain exactly m and n elements, respectively. What is the value of n?
(1) 7m=8n
(2) The intersection of M and N contains exactly 0.4m elements.
(2) NOT SUFFICIENT. This tells us that exactly 2/5th of the total number of elements in Set M are also common to Set N. However, it tells us nothing about the values of m and n.
Combined (1) and (2). NOT SUFFICIENT. From (1), we know that m must be a multiple of 8. From (2), we know, that m must be a multiple of 5 (in order to get an integer value for 2m/5). Thus m must be a multiple of 40 (LCM of 5 and 8). There are infinite number of multiples of 40. We cannot obtain a specific value of either m or n.
Answer choice: E
800 or bust!
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
Statement 1:Mo2men wrote:Sets M and N contain exactly m and n elements, respectively. What is the value of n?
(1) 7m=8n
(2) The intersection of M and N contains exactly 0.4m elements.
n = (7/8)m.
Since n must be an integer, m must be a MULTIPLE OF 8.
If m=8, then n = (7/8)(8) = 7.
If m=16, then n = (7/8)(16) = 14.
Since n can be different values, INSUFFICIENT.
Statement 2:
Since (0.4)m = (2/5)m, and the number of elements common to M and N must be an INTEGER, m must be a MULTIPLE OF 5.
If m=5, then the number of elements common to M and N = (2/5)(5) = 2.
Implication:
It's possible that M and N have exactly 2 elements in common, with the result that n could be any integer such that n≥2.
INSUFFICIENT.
Statements combined:
Since m must be both a multiple of 8 and a multiple of 5, m must be a MULTIPLE OF 40.
If m=40, then n = (7/8)(40) = 35.
If m=80, then n = (7/8)(80) = 70.
Since n can be different values, INSUFFICIENT.
The correct answer is E.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
-
- GMAT Instructor
- Posts: 2630
- Joined: Wed Sep 12, 2012 3:32 pm
- Location: East Bay all the way
- Thanked: 625 times
- Followed by:119 members
- GMAT Score:780
S1: n is some multiple of 7; NOT SUFFICIENT
S2: This only gives us the intersection; NOT SUFFICIENT
Together, we know that m is a multiple of 8 and divisible by 5. That doesn't tell us anything about the actual value of either number, so we're still stuck.
S2: This only gives us the intersection; NOT SUFFICIENT
Together, we know that m is a multiple of 8 and divisible by 5. That doesn't tell us anything about the actual value of either number, so we're still stuck.