用简便方法计算．
①7.28﹣（0.8+3.28）       ②73.8﹣1.64﹣13.8      ③13.25+5.82+6.75+4.18
④12.7﹣4.8﹣5.2           ⑤3.1+25.78+6.9         ⑥9.7+10.3﹣6.7．