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$\frac{20}{45} = \frac{4 \times 5}{9 \times 5} = \frac{4}{9} \\ \frac{12}{72} = \frac{1 \times 12}{6 \times 12} = \frac{1}{6} \\ \frac{136}{170} = \frac{4 \times 34}{5 \times 34} = \frac{4}{5} \\ \frac{21}{108} = \frac{7 \times 3}{36 \times 3} = \frac{7}{36} \\ \frac{24}{36} = \frac{2 \times 12}{3 \times 12} = \frac{2}{3} \\ \frac{15}{35} = \frac{5 \times 3}{5 \times 7} = \frac{3}{7} \\ \frac{102}{136} = \frac{6 \times 17}{8 \times 17} = \frac{6}{8} \\ \frac{90}{126} = \frac{18\times 5}{18\times7 } = \frac{5}{7} \\ \frac{3}{39} = \frac{1 \times 3}{13 \times 3} = \frac{1}{13} \\ \frac{36}{63} = \frac{9 \times 4}{9 \times 7} = \frac{4}{7} \\ \frac{21}{84} = \frac{1 \times 21}{4 \times 21} = \frac{1}{4} \\ \frac{151}{302} = \frac{151 \times 1}{151 \times 2} = \frac{1}{2} \\ \frac{130}{182} = \frac{26 \times 5}{26 \times 7} = \frac{5}{7} \\ \frac{490}{560} = \frac{70 \times 7}{70 \times 8} = \frac{7}{8} \\ \frac{10}{75} = \frac{2 \times 5}{15 \times 5} = \frac{2}{15}$