Thời gian đi nửa quãng đường đầu:
\(t_1=\frac{s}{2v_1}=\frac{s}{2.60}=\frac{s}{120}\left(h\right)\)
Thời gian đi nửa quãng đường sau:
\(t_2=\frac{s}{v_2}=\frac{s}{2v_2}\left(h\right)\)
Ta có: \(v_{tb}=\frac{s}{t_1+t_2}=\frac{s}{\frac{s}{120}+\frac{s}{2v_2}}=\frac{s}{s\left(\frac{1}{120}+\frac{1}{2v_2}\right)}=\frac{1}{\frac{1}{120}+\frac{1}{2v_2}}\)
\(\Leftrightarrow40=\frac{1}{\frac{1}{120}+\frac{1}{2v_2}}\\ \Leftrightarrow\frac{1}{40}=\frac{1}{120}+\frac{1}{2v_2}\\ \Leftrightarrow\frac{1}{2v_2}=\frac{1}{40}-\frac{1}{120}=\frac{1}{60}\\ \Leftrightarrow2v_2=60\\ \Leftrightarrow v_2=30\left(km/h\right)\)