If a, b, and c are positive, is ac > 5 ?
(1) a + b = 3
(2) 4=c-b
Algebra !
This topic has expert replies
- candygal79
- Senior | Next Rank: 100 Posts
- Posts: 49
- Joined: Mon Feb 10, 2014 1:18 am
- Followed by:3 members
- Bill@VeritasPrep
- GMAT Instructor
- Posts: 1248
- Joined: Thu Mar 29, 2012 2:57 pm
- Location: Everywhere
- Thanked: 503 times
- Followed by:192 members
- GMAT Score:780
From the stem, we need some information about how a and c relate to each other.
Statement 1 tells us that a + b = 3, but there's nothing about c. Not sufficient.
Similarly, 2 gives us information about c (4 = c - b), but there's nothing about A. Not sufficient.
Combined, we can rearrange each equation and solve.
a + b = 3
b = 3 - a
4 = c - b
b = c - 4
3 - a = c - 4
7 - a = c
7 = a + c
We still don't know if a*c > 5, though. If a = 3, then c = 4, and ac = 12. If a = .5, then c = 6.5, and ac = 6.5 * .5 = 3.25. We don't have a definitive answer.
Statement 1 tells us that a + b = 3, but there's nothing about c. Not sufficient.
Similarly, 2 gives us information about c (4 = c - b), but there's nothing about A. Not sufficient.
Combined, we can rearrange each equation and solve.
a + b = 3
b = 3 - a
4 = c - b
b = c - 4
3 - a = c - 4
7 - a = c
7 = a + c
We still don't know if a*c > 5, though. If a = 3, then c = 4, and ac = 12. If a = .5, then c = 6.5, and ac = 6.5 * .5 = 3.25. We don't have a definitive answer.
Join Veritas Prep's 2010 Instructor of the Year, Matt Douglas for GMATT Mondays
Visit the Veritas Prep Blog
Try the FREE Veritas Prep Practice Test
Visit the Veritas Prep Blog
Try the FREE Veritas Prep Practice Test
- Patrick_GMATFix
- GMAT Instructor
- Posts: 1052
- Joined: Fri May 21, 2010 1:30 am
- Thanked: 335 times
- Followed by:98 members
I would use logical deduction to solve.candygal79 wrote:If a, b, and c are positive, is ac > 5 ?
(1) a + b = 3
(2) 4=c-b
(1) doesn't tell us about c.
(2) doesn't tell us about a.
Together:
ac > 5 if we make c really big (statement 2 allows us to do this - no upper limit on c).
ac < 5 if we make a really tiny (0.0000001. Statement 1 allows this)
We cannot determine whether ac > 5. The answer is E
- Check out my site: GMATFix.com
- To prep my students I use this tool >> (screenshots, video)
- Ask me about tutoring.