1/9 : 7/12 = 1/9 · 12/7 = (1·4)/(3·7) = 4/21 = 4 : 21
1 5/16 : 4 3/8 = 21/16 : 35/8 = 21/16 · 8/35 = (3·1)/(2·5) = 3/10 = 3 : 10
