75*4.7+15.9*25
=25*(4.7*3+15.9)
=25*30
=750
6.11+9.22+8.33+7.44+5.55+4.56+3.67+2.78+1.89
=(6.11+1.89)+(9.22+2.78)+(8.33+3.67)+(7.44+4.56)+5.55
=8+12+12+12+5.55
=49.55
172.4*6.2+2724*0.38
=172.4*6.2+1000*0.38+1724*0.38
=172.4*(6.2+3.8)+380
=1724+380
=2104