42.5×6.28-4.25×2.8=4.25×62.8-4.25×2.8=4.25×（62.8-2.8）=4.25×60=255还有一题没回答呢50.9-7.3×6.950.9-7.3×6.9
=60-0.1-7.3×（7-0.1）
=60-0.1-7.3×7+7.3×0.1
=60-7.3×7+0.1×（7.3-1）
=60-51.1+0.63
=8.9+0.63
=9.53有点太复杂，还是采纳吧