The average per day income of A, B and C is $\text{Rs. }45$. If the average per day income of A and B is $\text{Rs. }40$ and that of B and C is $\text{Rs. }43$, then the per day income of B is:

Total daily income of A, B, and C = $45\times 3=\text{Rs. }135$ --- (Equation 1)
Total daily income of A and B = $40\times 2=\text{Rs. }80$ --- (Equation 2)
Total daily income of B and C = $43\times 2=\text{Rs. }86$ --- (Equation 3)

Adding Equation 2 and Equation 3:

$$\text{A}+2\text{B}+\text{C}=80+86=\text{Rs. }166$$

Subtracting Equation 1 ($\text{A}+\text{B}+\text{C}=135$) from this:

$$\text{B}=166-135=\text{Rs. }31$$