Why do we need to measure time? Name the types of devices used to measure time from ancient times to present time. Which one of them gives the most accurate time? 

An object moves at $10 \mathrm{km}/\mathrm{h}$for $10$ minutes and then moves $15 \mathrm{km}/\mathrm{h}$ for$15$ minutes. Find:

Total distance travelled by the object

 Average speed of the object

A bus takes $15 \mathrm{hours}$ to travel from town X and Y and $12 \mathrm{hours}$ to travel from town Y to Z.

Calculate the speed of the bus from town 'X' to town 'Y'.