Bonsoir, ces fractions ne sont pas irréductibles car elles ont toutes un facteur commun:
15/20 = 15:5/20:5 = 3/4
240/303 = 240:3/303:3 = 80/101
42/52 = 42:2/52:2 = 12/26 = 12:2/26:2 = 6/13
324/420 = 324:3/420:3 = 108/140 = 108:2/140:2 = 54/70 = 54:2/70:2 = 27/35
90/45 = 90:5/45:5 = 18/9 = 18:9/9:9 = 2/1
18/12 = 18:3/12:3 = 6/4 = 6:2/4:2 = 3/2
