(1999×99+2000×100+1999+2000-1900)÷4000
=(1999×(99+1)+2000×100+100)÷4000
=((2000-1)×100+2000×100+100)÷4000
=(2000×100×2+100-100)÷4000
=2000×2×100÷4000
=100