ANSWER:
B. $80
EXPLANATION:
[tex]b(x)=\begin{cases}0.15x,{x\leq400} \\ 0.10(x-400)+60,x>400{}\end{cases}[/tex]We'll use the below function to determine how much is the bill for a person who uses 600 kWh in a month;
[tex]\begin{gathered} b(x)=0.10(x-400)+60 \\ b(600)=0.10(600-400)+60 \\ =0.10(200)+60 \\ =20+60 \\ =\text{\$}80 \end{gathered}[/tex]So the bill is $80