gọi chiều dài là a; chiều rộng là b
$⇒\left \{{{2(a+b)=340} \atop {3a-4b=20}}\right.$
$⇒\left \{{{4a+4b=680} \atop {3a-4b=20}}\right.$
$⇒\left \{{{7a=700} \atop {3a-4b=20}}\right.$
$⇒\left \{{{a=100} \atop {3a-4b=20}}\right.$
$⇒\left \{{{a=100} \atop {4b=280}}\right.$
$⇒\left \{{{a=100} \atop {b=70}}\right.$
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