Question 1189258
.
one third of a certain journey was covered at the speed of 20 km per hour , 
one fourth at the rate of 30 km per hour and the rest at the rate of 50 km per hour.
Find the average speed ( per hour ) of the whole journey ?
~~~~~~~~~~~~~~~~



            Calculations by @josgarithmetic have  FATAL  ERRORS, 

            so I came to bring a correct solution.



<pre>
Let d be the distance.


Part {{{d/3}}} was covered at 20 km/h;  it took  {{{d/(3*20)}}} = {{{d/60}}} hours.

Part {{{d/4}}} was covered at 30 km/h;  it took  {{{d/(4*30)}}} = {{{d/120}}} hours.


The rest distance,  {{{d - d/3 - d/4}}} = {{{(5d)/12}}}  kilometers, was covered at 50 km/h;  

                    it took {{{(5d)/(12*50)}}} = {{{d/120}}} hours.


Total time is  {{{d/60 + d/120 + d/120}}} = {{{d/60 + d/60}}} = {{{2d/60}}} = {{{d/30}}}.


Therefore, the average speed = {{{d/((d/30))}}} = 30 km/h.    <U>ANSWER</U>.  
</pre>

Solved.



//////////////



Do not accept any other answer.