[tex]v=\dfrac{d}{t}\to d=vt\to t=\dfrac{d}{v}\\\\d_1=2100km;\ v_1=800km/h\\\\t_1=\dfrac{2100}{800}h=\dfrac{21}{8}h\\\\d_2=1800km;\ v_2=1000km/h\\\\t_2=\dfrac{1800}{1000}h=\dfrac{18}{10}h\\\\The\ average\ speed:v=\dfrac{d_1+d_2}{t_1+t_2}\\\\d_1+d_2=2100km+1800km=3900km\\\\t_1+t_2=\dfrac{21}{8}h+\dfrac{18}{10}h=\dfrac{21\cdot5}{8\cdot5}h+\dfrac{18\cdot4}{10\cdot4}h=\dfrac{105}{40}h+\dfrac{72}{40}h=\dfrac{177}{40}h\\\\v=\dfrac{3900}{\frac{177}{40}}km/h=3900\cdot\dfrac{40}{177}km/h\approx\boxed{881\ km/h}[/tex]
