0.9+0.09+0.009+0.0009
=(1-0.1)+(0.1-0.01)+(0.01-0.001)+(0.001-0.0001)
=1+(0.1-0.1)+(0.01-0.01)+(0.001-0.001)-0.0001
=1-0.0001
=0.9999
0.9+0.09+0.009+0.0009
=(1-0.1)+(0.1-0.01)+(0.01-0.001)+(0.001-0.0001)
=1+(0.1-0.1)+(0.01-0.01)+(0.001-0.001)-0.0001
=1-0.0001
=0.9999