(A+B+C)/3=45kg

=>[(A+B)+C]=135kg..................................(i)

(A+B)/2=40kg

(A+B)=80kg.............................................(ii)

(B+C)/2=43kg

=>C=86-B..............................................(iii)

Now,

[(A+B)+C]=135

[(80)+(86-B)]=135.............put values from (ii)& (iii) into (i)

B=31kg

Ans weight of B is 31 kg