$V_{tb}=\frac{S_1+S_2}{t_1+t_2}=\frac{2S_1}{t_1+t_2}(do :S_1=S_2)$$\Leftrightarrow V_{tb}=\frac{2S_1}{\frac{S_1}{v_1}+\frac{S_1}{v_2}}=\frac{2}{\frac{1}{v_1}+\frac{1}{v_2}}$
Do$: V_{tb}=8;v_1=12$nên:
$\Leftrightarrow8=\frac{2}{\frac{1}{12}+\frac{1}{v_2}}\Leftrightarrow v_2=6$