M=（6.875+/-0.02)kg=( 6875+/-20)gL=（8.54+0.02）cm=（85.4+0.2）mm=（0.0854+0.0002）mP=（1.293+0.005）mg/cm3=(1.293+0.005)kg/m3
1kg = 1000g =1000,000mg1cm = 10mm = 0.01m1mg/cm3 = 0.000001kg/0.000001m3 = 1kg/m3
