Question

A train travels the first 15km at uniform speed of 30km/h next 75 km at uniform speed of 50 km/h at last 10 km at uniform speed of 20 km/h . the average speed for the entire journey of the train ?

Answer 1

Helo....

speed = distance /time

average speed = total distance upon total time


total time = 
distance upon speed  in first time
+
distance upon speed  of 2 nd time
+
distance upon speed  of 3 rd time


so

= 15/30  + 75/50 + 10 /20

= 0.5 + 1.5 + 0.5

  2.5 hour

total distance=  15+75+10 =100

average speed = total distance upon total time

= 100/2.5
= 40

thank you...

☺

Answer 2

Here is your solution

Given :-

First 15 km at a uniform speed of 30km/h.

Next 75km at at a uniform speed of 50km/h.

Last 10km/h at a uniform speed of 20km/h.

Time 1st = distance /speed = 15/30
Time 2nd = distance /speed = 75/50
Time 3rd = distance /speed = 10/20

$total \: time = \frac{15}{30} + \frac{75}{50} + \frac{10}{20} \\ \\ total \: time= \frac{150 + 450 + 150}{300} \\ \\ total \: time \: = \frac{750}{300} = \frac{5}{2}$
Total distance = 15km+75km+10km = 100km

$average \: speed = \frac{total \: distance}{total \: time} \\ average \: speed = \frac{100}{ \frac{5}{2} } = \frac{100 \times 2}{5}\\average \: speed = 40km/h$

Hence,
Average speed of train is 40km/h

Hope it helps you