a) (1 si 5/7) × (4 si 5/18) = (7×1+5)/7 × (4×18+5)/18 = 12/7 × 77/18 = 2/1 × 11/3 = 22/3
b) (4 si 1/5) × (3 si 3/14) = (4×5+1)/5 × (3×14+3)/14 = 21/5 × 45/14 = 3/1 × 9/2 = 27/2
c) 17/9 - 4/27 × (3 si 15/16) = 17/9 - 4/27 × (3×16+15)/16 = 17/9 - 4/27 × 63/16 = 17/9 - 1/3 × 7/4 = 17/9 - 7/12 = 68/36 - 21/36 = 47/36