3.5×25×1.25
=3.5×100÷2÷2×1.25
=3.5÷2÷2×1.25×100
=1.75÷2×125
=0.875×125
=（0.8+0.07+0.005）×125
=100+8.75+0.625
=109.375
18×15/19
=（19-1）×15/19
=19×15/19-15/19
=15-15/19
=14又4/19