ta có: \(\dfrac{1}{4}\left(v_1+v_2\right)=\dfrac{1}{2}\left(v_1-v_2\right)\) ( quãng đường đi là như nhau )
\(\Rightarrow3v_2=v_1\)
\(AB=\dfrac{1}{4}\left(v_1+v_2\right)=\dfrac{1}{2}\left(v_1-v_2\right)=v_2\)
Tắt máy: \(AB=v_2t\Rightarrow t=\dfrac{AB}{v_2}=\dfrac{v_2}{v_2}=1\left(h\right)\)