2.1*3.6*1.6*8.4
=(2+0.1)*3.6*1.6*(10-1.6)
=(7.2+0.36)*(16-2.56)
=7.56*13.44
=(8-0.44)*(13+0.44)
=104-5*0.44-0.44*0.44
=104-0.44*5.44
=104-2.3936
=101.6064