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Answer:
3
Step-by-step explanation:
7a+3b=20
3a+7b=10
Substitution:
[tex]\begin{bmatrix}3\cdot \frac{20-3b}{7}+7b=10\end{bmatrix}[/tex]
[tex]a=\frac{20-3\cdot \frac{1}{4}}{7}[/tex]
[tex]a=\frac{11}{4},\:b=\frac{1}{4}[/tex]
11=4 + 1=4 = 12/4
12/4 = 3