Define 'average speed'. An object moves with a uniform speed of $10\, m s^{-1}$ for $5\, s$ and then with a uniform speed of $5\, m s^{-1}$ for $10\, s$. Find its average speed.

$(6.67\, m s^{-1})$ Average speed of an object is defined as the total distance travelled divided by the total time taken.
Step $1$: Calculate the total distance covered.
Distance for the first part $= 10\, m s^{-1} \times 5\, s = 50\, m$.
Distance for the second part $= 5\, m s^{-1} \times 10\, s = 50\, m$.
Total distance $= 50\, m + 50\, m = 100\, m$.
Step $2$: Calculate the total time taken.
Total time $= 5\, s + 10\, s = 15\, s$.
Step $3$: Calculate the average speed.
Average speed $= \frac{\text{Total distance}}{\text{Total time}} = \frac{100\, m}{15\, s} \approx 6.67\, m s^{-1}$.