The average weight of A, B and C is 42 kg. If the average weight of A and B be 40 kg and that of B and C be 43 kg, then the weight of B is: Choose the correct choice from the following answers:
A) 33 kg
B) 31 kg
C) 32 kg
D) 40 kg
E) 37 kg

Rationale: "let a , b , c represent their respective weights . then , we have : a + b + c = ( 42 x 3 ) = 126 . . . . ( i ) a + b = ( 40 x 2 ) = 80 . . . . ( ii ) b + c = ( 43 x 2 ) = 86 . . . . ( iii ) adding ( ii ) and ( iii ) , we get : a + 2 b + c = 166 . . . . ( iv ) subtracting ( i ) from ( iv ) , we get : b = 40 . b ' s weight = 40 kg . d"
Answer: D