A car covered 150 km in 5 hours. If it travels at one-third its usual speed, then how much more time will it take to cover the same distance?

$$\eqalign{ & \underleftrightarrow {\,\,\,\,\,\,\,\,\,\,150{\text{ km}}\,\,\,\,\,\,\,\,\,\,} \cr & {\text{Initial speed}} = \frac{{150}}{5} = 30{\text{ km/h}} \cr & {\text{New speed}} = \frac{1}{3} \times 30 = 10{\text{ km/h}} \cr & {\text{Time taken to cover 150 km}} = \frac{{150}}{{10}} = 15{\text{ h}} \cr & {\text{Time difference}} = 15 - 5 = 10{\text{ hour}} \cr} $$