5.9+ 8.99 +12.999+ 0.111
=5.9+ 8.99 +12.999+0.1+0.01+0.001
=5.9+0.1+8.99+0.01+12.999+0.001
=（5.9+0.1）+（8.99+0.01）+（12.999+0.001）
=6+9+13
=15+13
=28