a) 2 × (5 × 89) = (2 × 5) × 89 = 10 × 89 = 890
b) 645 × 2 × 50 = 645 × (2 × 50) = 645 × 100 = 64 500
c) 125 × 457 × 8 × 10 = (125 × 8) × (457 × 10) = 1000 × 4570 = 4 570 000