If a - b = 4 and a2 + b2 = 30, find the value of ab. Answers:
A) 7
B) 12
C) 15
D) 18
E) 20

Rationale: "explanation : 2 ab = ( a 2 + b 2 ) - ( a - b ) 2 = 30 - 16 = 14 ab = 7 answer : a"
Answer: A