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**Hint:**In this question we will use the basic expression of average speed to find the result. First we will calculate the total time and total distance travelled by the object and then by substituting these values in the average speed expression will give us the answer. Further we will discuss the basics of speed.

**Formula used:**

${s_{avg}} = \dfrac{d}{t}$

**Complete answer:**

Here, as we already know, the average speed is given by the ratio of total distance traveled by the object and the total time taken by the object to travel that particular distance.

We first calculate the time for first 10km will be given by:

${t_1} = \dfrac{{10 \times {{10}^3}}}{{100}} = 100\sec $

Now, the time for the last 10km will be given as:

${t_2} = \dfrac{{10 \times {{10}^3}}}{{50}} = 200\sec $

From above results, we calculate the total time as:

$100 + 200 = 300\sec $

Now, we know that the total distance is given as:

$10 + 10 = 20km$

At last, we use the above values to find the average speed of the object, which is given by:

$\eqalign{& {s_{avg}} = \dfrac{{20000}}{{300}} \cr

& \therefore {s_{avg}} = 66.7m/\operatorname{s} \cr} $

**Therefore, the correct option is C) i.e., the average speed of the object is 66.7m/s.**

**Additional information:**

We already know the basic difference of speed and velocity i.e., speed is the measure of how fast an object can travel, whereas velocity tells us the direction of this speed. Speed is a scalar quantity that means it has only magnitude, whereas velocity is a vector quantity that means it has both magnitude and direction. The S.I unit of velocity is meter per second (m/sec).

Also, we know that the supersonic speed is the speed of an object that exceeds the speed of sound. In an object which is traveling in dry air at a temperature of 20 °C or 68 °F at sea level, this speed is approximately 343.2 m/s. Further, speed that is greater than five times the speed of sound is referred to as hypersonic.

Further we can also understand this concept by observing the vehicles that are flying at supersonic speeds are flying faster than the speed of sound. The speed of sound is about 768 miles per hour or 1236 kilometers per hour, at sea level. These speeds are called the Mach numbers. The Mach number is defined as the ratio of the speed of the aircraft to the speed of the sound. Flight that is faster than Mach 1 is termed as supersonic. Supersonic includes speeds up to five times faster than the speed of the sound, or Mach 5.

**Note:**

We should remember that speed is a scalar quantity, which means it does not have any direction but has only magnitude. Speed depends on distance travelled by an object and time taken by the object to travel that distance.

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