5. $\frac{3}{10}=\frac{b + 7}{20}$

Explanation:

Step1: Cross - multiply

$3\times20 = 10\times(b + 7)$

Step2: Simplify left - hand side

$60=10\times(b + 7)$

Step3: Distribute on right - hand side

$60 = 10b+70$

Step4: Subtract 70 from both sides

$60 - 70=10b+70 - 70$ $- 10 = 10b$

Step5: Divide both sides by 10

$\frac{-10}{10}=\frac{10b}{10}$ $b=-1$

Answer:

$b = - 1$