Perimeter = 2(L + B) and L : B = 3 : 1.<br><br>
&bull; Let breadth = 1 unit, then length = 3 units<br>
&bull; Perimeter = 2(3 + 1) = 8 units<br>
&bull; 8 units must be between 70 and 80<br>
&bull; 70 ÷ 8 = 8.75, 80 ÷ 8 = 10<br>
&bull; So 1 unit must be 9 cm (the only whole number between 8.75 and 10)<br>
&bull; Length = 3 × 9 = 27 cm<br>
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Final Answer: 27