3 = 3 * 1;
$4 = 2^2$;
$5 = 5 * 1$;
$9 = 3^2$;
$20 = 2^2 * 5$;
$НОК(3;4;5;9;20) = 2^2 * 3^2 * 5 = 4 * 9 * 5 = 9 * 20 = 180$;
$\frac{3}{4} = \frac{45 * 3}{45 * 4} = \frac{135}{180}$;
$\frac{9}{20} = \frac{9 * 9}{9 * 20} = \frac{81}{180}$;
$\frac{3}{5} = \frac{36 * 3}{36 * 5} = \frac{108}{180}$;
$\frac{2}{3} = \frac{60 * 2}{60 * 3} = \frac{120}{180}$;
$\frac{5}{9} = \frac{20 * 5}{20 * 9} = \frac{100}{180}$;
$\frac{81}{180} < \frac{100}{180} < \frac{108}{180} < \frac{120}{180} < \frac{135}{180}$;
$\frac{9}{20} < \frac{5}{9} < \frac{3}{5} < \frac{2}{3} < \frac{3}{4}$.