`1/3s` đầu: $v_1=20\ km/h$
`2/3s` sau: $v_2=36\ km/h$
$v_{tb}=\ ?\ km/h$
Áp dụng công thức:
$v_{tb}=\dfrac{s_1+s_2}{t_1+t_2}=\dfrac{s}{\dfrac{s_1}{v_1}+\dfrac{s_2}{v_2}}=\dfrac{s}{\dfrac{\dfrac{s}{3}}{20}+\dfrac{\dfrac{2s}{3}}{36}}=\dfrac{s}{\dfrac{s}{60}+\dfrac{s}{54}}=28,4\ (km/h)$