Four identical triangles are cut from a 42 cm by 18 cm rectangle.<br><br>Area of rectangle = 42 x 18 = 756 cm&sup2;<br>Area cut away = 84 cm&sup2; (4 triangles).<br>Fraction cut away = <span class="math-inline">\(\frac{84}{756}\)</span> = <span class="math-inline">\(\frac{1}{9}\)</span>.<br><br>Final Answer: <span class="math-inline">\(\frac{1}{9}\)</span>