a) 7 × 5 - 7 × 3 = 7 × ( 5 - 3 ) = 7 × 2 = 14
b) 2 × 3 + 3 × 8 - 3 = 3 × ( 2 + 8 - 1 ) = 3 × 9 = 27
c) 15 + 15 × 7 = 15 × ( 1 + 7 ) = 15 × 8 = 120
d) 7 × 9 - 7 = 7 × ( 9 - 1 ) = 7 × 8 = 56
e) 143 × 81 + 143 × 11 + 143 × 7 + 143 = 14 300
= 143 × ( 81 + 11 + 7 + 1 ) =
= 143 × 100 =
= 14 300