Question 1189258
{{{1-1/3-1/4}}}

{{{12/12-4/12-3/12}}}

{{{5/12}}}, the rest at 50 kilom per hr


<pre>
          SPEEDS      TIME                  DISTANCE

PART1        20       (d/3)(1/20)               d/3

PART2        30       (d/4)(1/30)               d/4

PART3        50       (5d/12)(1/50)             (5/12)d

SUM                                               d
</pre>

total distance, d

total time,  {{{d/60+d/120+(5d)/600}}}


AVERAGE SPEED,  {{{cross(1/(1/60+1/20+5/600))}}}
               {{{1/(1/60+1/120+5/600)}}}
-

{{{1/(1/60+1/120+1/120)}}}

{{{1/(1/60+2/120)}}}

{{{1/(1/60+1/60)}}}

{{{1/(2/60)}}}

{{{1/(1/30)}}}

{{{highlight_green(30)}}}