Question

A car travels 30 km at a uniform speed of 40 km per hour and the next 30 km at a uniform speed of 20km per hour find its average speed

Answer 1

given that ,
    1st 30 km uniform speed is (u1) = 40 km/h 
     2nd 30 km uniform speed is ( u2 ) = 20 km/h
     total distance ( d ) = 30 + 30 = 60 km
     time taken in 1st time = d / t = 30/40 also, 3h / 4 = 180/4 = 45 min
     tim taken in 2nd time = d/t = 30/20 also, 3h/2 = 180/2 = 90 min
    total time = 45 + 90 = 135 min = 135/60 h
av.speed = 60/135/60 = 60 * 60 /135 = 26.66 km/h ( answer )    
  
 
 
    

Answer 2

Given that ,  

$1^{st}$  $30 km$ uniform speed is $(u1)$ $= 40 \frac{km}{h}$

$2^{nd}$ $30 km$ uniform speed is $(u2)$ $= 20 \frac{km}{h}$

Total distance $( d )$ $= 30 + 30 = 60 km$

Time taken in $1^{st}$ time $= \frac{d}{t} = \frac{30}{40}$  also, $\frac{3h}{4} = \frac{180}{4} = 45 min$

Tim taken in $2^{nd}$ time $= \frac{d}{t} = \frac{30}{20}$  also, $\frac{3h}{2} = \frac{180}{2} = 90 min$

Total time $= 45 + 90 = 135 min = \frac{135}{60} h$

Average speed $= \frac{60}{135}{60} = 26.66 \frac{km}{h}$

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