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4. 电器商场某种型号的电视机连续两次降价促销。第一次降价279元，第二次降价121元。这台电视机现在卖多少元？
答案
4. 3888 - 279 - 121 = 3888 - (279 + 121) = 3888 - 400 = 3488(元)
5. 某修路队计划修一条4千米长的公路，计划分四次修完。如果要按计划完成，修路队第三次要修多少米？
答案
5. 4千米 = 4000米，4000 - 343 - 748 - 2657 = 4000 - (343 + 2657) - 748 = 4000 - 3000 - 748 = 1000 - 748 = 252(米)
6. 用简便方法计算下面各题。
(1)$254-(154-89)$ (2)$9999+999+99+9$
(3)$2+4+6+8+10+···+30-1-3-5-7-9-···-29$
(1)$254-(154-89)$ (2)$9999+999+99+9$
(3)$2+4+6+8+10+···+30-1-3-5-7-9-···-29$
答案
6. (1)254 - (154 - 89) = 254 - 154 + 89 = 100 + 89 = 189
(2)9999 + 999 + 99 + 9 = (10000 - 1) + (1000 - 1) + (100 - 1) + (10 - 1) = 11110 - 4 = 11106
(3)2 + 4 + 6 + 8 + 10 + ······ + 30 - 1 - 3 - 5 - 7 - 9······ - 29 = (2 - 1) + (4 - 3) + (6 - 5) + ······ + (30 - 29) = 1 × (30 ÷ 2) = 15
(2)9999 + 999 + 99 + 9 = (10000 - 1) + (1000 - 1) + (100 - 1) + (10 - 1) = 11110 - 4 = 11106
(3)2 + 4 + 6 + 8 + 10 + ······ + 30 - 1 - 3 - 5 - 7 - 9······ - 29 = (2 - 1) + (4 - 3) + (6 - 5) + ······ + (30 - 29) = 1 × (30 ÷ 2) = 15
