2.5×0.999×0.4=(2.5×0.4)×0.999=1×0.999=0.999
把0.4看成4/10，先用2.5*4=10/10=1，再成0.999就简单啦，结果就是0.999
2.5×0.999×0.4=0.25x4x0.999=1x0.999=0.999
=2.5×0.4x0.999=1x0.999=0.999
2.5*0.4=12.5×0.999×0.4=0.999
