t = \frac{400 m - 100 m}{25 k m / h + 20 k m / h} = 6, 7 s

${t = \frac{{400}\,\mathrm{m} - {100}\,\mathrm{m}}{{25}\,\mathrm{km/h} + {20}\,\mathrm{km/h}} = {6,7}\,\mathrm{s} \quad}$

Error