17.2÷2+2.8×0.5
=17.2×0.5+2.8×0.5
=（17.2+2.8）×0.5
=20×0.5
=10
1.72÷4+2.28×0.25
=1.72×0.25+2.28×0.25
=（1.72+2.28）×0.25
=4×0.25
=1