a) 23(145-17)-145(23-6)
= 23.145 - 17.23 - 145 . 23 + 6 . 145
= 145.6 - 17.23
= 870 - 391 = 479
b) 24(15-4)+4(24-15)
= 25.15 - 25.4 + 4.24 - 15.4
= 15(25-4) - 4(25-24)
= 15.21 - 4.1
= 311
c) 199(15-17)-199(-17+5)
= 199 [ 15 - 17 - (-17 + 5 )]
= 199 .(15 - 17 + 17-5 )
= 199 . 10
= 1990
d) -39(5-99)+99(10-39)
= -39.5 + 39.99 + 99.10 - 99.39
= -39 . 5 + 99.10
= -195 + 990
= 795