7.5*4-3.9=(5+2.5)*4-(4-0.1)=5*4+2.5*4-4+0.1=20+10-4+0.1=26.1
7.5 X 4 - 3.9= 3 X 2.5 X 4 - 3.9= 3 X 10 - 3.9= 30 - 3.9= 26.1
7.5分成(5+2.5) 3.9分成(4-0.1)