脱式计算(分能简算的要简算.)
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(1)70.8-1.25-1.75, =70.8-(1.25+1.75), =70.8-3, =67.8; (2)5.81+8.3+4.19+11.7, =(5.81+4.19)+(8.3+11.7), =10+20, =30; (3)8.7×9.9, =8.7×(10-0.1), =8.7×10-8.7×0.1, =87-0.87, =86.13; (4)0.25×17.3×0.4, =0.25×0.4×17.3, =0.1×17.3, =1.73; (5)0.15×9+0.15, =0.15×(9+1), =0.15×10, =1.5; (6)3.54×0.29-0.29×2.54, =(3.54-2.54)×0.29, =1×0.29, =0.29. |
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后,新数比原数少138.6,原数是多少?