Tính.

a) 36 × 40 = (36 × 4) × 10 = 1 440               

72 × 60 = (72 × 6) × 10 = 4 320                                                 

89 × 50 = (89 × 5) × 10 = 4 450

b) 450 × 70 = (45 × 7) × 100 = 31 500                   

2 300 × 50 = (23 × 5) × 1 000 = 115 000

17 000 × 30 = (17 × 3) × 10 000 = 510 000