\frac{v_1t_1+v_2t_2+\cdots{}+v_nt_n}{t_1+t_2+\cdots{}+t_n}=\frac{4\frac{km}{h}\cdot{}x + 6\frac{km}{h}\cdot{}\frac{2}{3}x}{x+\frac{2}{3}x}=4.8