$${\Delta t = \frac{s}{v_i} - \frac{s}{v_b} = \frac{{0,3}\,\mathrm{km}}{{5}\,\mathrm{km/h}} - \frac{{0,3}\,\mathrm{km}}{{30}\,\mathrm{km/h}} = {0,05}\,\mathrm{h} = {3}\,\mathrm{min}.}$$