a, 15 . 12 - 3 . 5 . 10
= 15 . 12 - 15 . 10
= 15 . ( 12 - 10 )
= 15 . 2
= 30
b, 45 - 9 . ( 13 + 5 )
= 45 - 9 . 13 - 9 . 5
= 45 - 45 - 117
= 0 - 117
= -117
c, 29 . ( 19 - 13 ) - 19 . ( 29 - 13 )
= 29 . 19 - 29 . 13 - 19 . 29 + 19 . 13
= ( 29 . 19 - 19 . 29 ) + 13 . ( 19 - 29 )
= 0 + 13 . ( -10 )
= -130