用简便方法计算19/20×7%+7/20×81% 4/7×3%+1/7×4%+1/7×12% 19/20×7%+7/20×81% =7/20*19%+7/20*81%=7/20(19%+81%)=7/20*100%=7/204/7×3%+1/7×4%+1/7×12% =1/7*12%+1/7*4%+1/7*12%=1/7(12%+4%+12%)=1/7*28%=4...