Answer (D): Notice that the difference between 100 and 87 is 13, a prime number. This fact will help to simplify the problem. Subtract the second equation from the first to get
13=(ab+c)−(bc+a)=ab−bc−a+c=b(a−c)−(a−c)=(b−1)(a−c).
Thus b−1=±1 or b−1=±13.
* If b−1=−1, then b=0, implying c=100, a=87, and ca=60, which is impossible.
* If b−1=1, then b=2 and a−c=13, implying ca=58=2⋅29, which cannot be true if a−c=13.
* If b−1=13, then b=14 and a−c=1, implying ca=46=2⋅23, which cannot be true if a−c=1.
* If b−1=−13, then b=−12 and a−c=−1, implying ca=72, which is satisfied when a=−9 and c=−8. In fact, a=−9, b=−12, and c=−8 satisfies all three equations.
The requested value is ab+bc+ca=(−9)(−12)+(−12)(−8)+(−8)(−9)=108+96+72=276.