50.99—17.11 39.01—23.89 怎样简便计算

50.99-17.11
=51-0.01-17-0.11
=51-17-(0.01+0.11)
=34-0.12
=33+1-0.12
=33+0.88
=33.88
39.01-23.89
=39+0.01-24+0.11
=25+0.01+0.11
=25+0.12
=25.12
50.99—17.11+ 39.01—23.89=33.88+25.12=49