the answer is -$97 for sure.  
" z- x3 e. x: N$ `TVBNOW 含有熱門話題,最新最快電視,軟體,遊戲,電影,動漫及日常生活及興趣交流等資訊。Think about it this way  
; l- m+ e  ^3 j3 v% A5 q! Y公仔箱論壇1) if the 100 bucks is not a fake one, the store owner will gain $3 ($21-$18) 
/ x# o& V3 D# itvb now,tvbnow,bttvb2) Now, the owner discovered that the 100 bucks is fake. You can think this as a lost of $100. therefore the overall gain is $3-$100 = -$97 |