Question

a car travels the first 20km with a speed of 40kmh-1 and the next 40km with a speed of 80kmh-1.find the speed​

Answer 1

Correct Question: A car travels the first 20km with a speed of 40kmh-1 and the next 40km with a speed of 80kmh-1. Find the average speed.

Answer:

$\large{\underline{\boxed{\sf 60 \: kmh^{-1}}}}$

Explanation:

For first 20 km,

Speed of the car = 40 kmh⁻¹

Distance travelled = 20 km

Time taken = $\dfrac{\text{Distance}}{\text{Speed}}$

⇒ Time = $\dfrac{20}{40}$

⇒ Time = $\dfrac{1}{2}$ hour

⇒ Time = 30 minutes

For next 40 km,

Speed of car = 80 kmh⁻¹

Distance travelled = 40 km

Time taken = $\dfrac{\text{Distance}}{\text{Speed}}$

⇒ Time = $\dfrac{40}{80}$

⇒ Time = $\dfrac{1}{2}$ hour

⇒ Time = 30 minutes

__________________________________

Now,

Total distance travelled = (20 + 40) km

⇒ Distance = 60 km

Total time taken = (30 + 30) minutes

⇒ Time taken = 1 hour

$\bf Average \: Speed = \frac{Total \: distance}{Total \: time \: taken}$

$\implies \sf Average \: Speed = \frac{60}{1} \\ \\ \implies \sf Average \: Speed = 60 \: kmh {}^{ - 1}$

Hence, the average speed is 60 kmh⁻¹.

Answer 2

Question

A car travels the first 20 km with a speed of 40 km/h-1 and the next 40 km with a speed of 80 km/h-1. Find the average speed.

                                                                                                             

SOLUTION

First Case

Given,

Speed of car = $40\:kmh^-^1$

Distance travelled by car = 20 km

Time = ?

Now we have to find time.

$Speed=\dfrac{Distance}{Time}\\\\\\Time=\dfrac{Distance}{Speed}\\\\\\Time=\dfrac{20}{40}\\\\\\Time=\dfrac{1}{2}\:h\:\\\\\\=\boxed{\bold{30\:min.}}$

                                                                                                   

Second Case

Given,

Speed of car = $80\:kmh^-^1$

Distance travelled by car = 40 km.

Time = ?

Now we have to again find time.

$Time=\dfrac{Distance}{Speed}\\\\\\Time=\dfrac{40}{80}\\\\\\Time=\dfrac{1}{2}\:h\\\\\\Time=\boxed{\bold{30\:min.}}$

                                                                                                     

Now we are going to find average speed.

$\boxed{Average\:Speed=\dfrac{Total\:distance\:covered}{Total\:time\:taken}}$

Total Distance Travelled = 20 + 40 = 60 km

Total Time Taken = 30 + 30 = 60 min = 1 h

Now we come on given formula

$Average\:Speed =\boxed{\bold{\dfrac{60}{1}\\\\\\Average\:Speed=60\:kmh^-^1}}$