Tính bằng cách thuận tiện (theo mẫu).

a) 5 × 3 × 4 = (5 × 4) × 3

                   = 20 × 3

                   = 60

b) 6 × 5 × 7 = 30 × 7

                    = 210

c) 20 × 9 × 5 = (20 × 5) × 9

                     = 100 × 9

                     = 900

d) 2 × 7 × 50 = (2 × 50) × 7

                      = 100 × 7

                      = 700