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.

candygal79 wrote:If a, b, and c are positive, is ac > 5 ?

(1) a + b = 3

(2) 4=c-b

I would use logical deduction to solve.

(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