19.88+9.9+198.8
=19.88+19.88×10+9.9
=19.88×（1+10）+9.9
=19.88×11+9.9
=19.88×11+0.9×11
=11×（19.88+0.9）
=11×20.78
=（10+1）×20.78
=20.78×10+20.78
=207.8+20.78
=228.58