72.
а)8,91+25,7+1,09=8,91+1,09+25,7=10+25,7=35,7
б)6,64+7,12+2,88=7,12+2,88+6,64=10+6,64=16,64
в)7,15-9,42+12,85-0,58=7,15+12,85-9,42+0,58=20-10=10
г)18,9-6,8-5,2-4,1=18,9-5,2-6,8-4,1=13,7-2,7=11
77.
а)3,5·6,8+3,5·3,2=3,5·(6,8+3,2)=3,5·10=35
б)12,4·14,3-12,4·4,3=12,4·(14,3-4,3)=12,4·10=124