tonebeeze wrote:If a, b, c, and d are integers, is ab - bd - ac + cd < 0?
1. b < c
2. d < a
ab - bd - ac + cd
= b(a-d) - c(a-d)
= (a-d)(b-c)
Question rephrased: Is (a-d)(b-c) < 0?
Statement 1: b<c.
Thus, (b-c) = smaller - larger = negative.
No information about (a-d).
Insufficient.
Statement 2: d<a.
Thus, (a-d) = larger - smaller = positive.
No information about (b-c).
Insufficient.
Statements 1 and 2 combined:
(a-d)(b-c) = positive * negative = negative.
Sufficient.
The correct answer is
C.
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