Question
21 – 3 (b - 7) = b+ 20
✓
Answer
$\begin{aligned} & 21-3(b-7)=b+20 \\ & \Rightarrow 21-3 b+21=b+20 \\ & \Rightarrow-3 b+42=b+20 \\ & \Rightarrow-3 b-b=20-42 \\ & \Rightarrow-4 b=-22 \\ & \Rightarrow b=\frac{-22}{-4}=\frac{11}{2} \\ & \therefore b=\frac{11}{2}=5 \frac{1}{2}\end{aligned}$
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