A car moves with a speed of 40 km/h for the first hour and then with a speed of 60 km/h for the next half hour and finally with a speed of 30 km/h for the next 1.5 hours. Calculate the average speed of the car. [2 MARKS]

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Solution

Finding $s_{1}, s_{2}, s_{3} :$ 1 Mark
Final Answer: 1 Mark

Distance travelled in first hour, $s_{1} = s p e e d \times t i m e = 40 k m / h r \times 1 h r = 40 k m$

Distance travelled in next half an hour, $s_{2} = s p e e d \times t i m e = 60 k m / h r \times 0.5 h r = 30 k m$

Distance travelled in last hour, $s_{3} = s p e e d \times t i m e = 30 k m / h r \times 1.5 h r = 45 k m$

Thus, total distance travelled = $s_{1} + s_{2} + s_{3} = (40 + 30 + 45) k m = 115 k m$

Total time taken = 3 hours

Hence, $a v e r a g e s p e e d = \frac{T o t a l D i s t a n c e}{T o t a l T i m e T a k e n} = 38.33 k m / h r$

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