Question 923113
Average speed for the whole trip   = 75 kph
total journey time for whole trip =  1 hr
Distance covered in whole trip  =  speed *time
                                 = 75*1
                                  = 75 km
Distance covered in first 20 minutes with speed 80 kph  = {{{80*(20/60)}}}
                                                        ={{{80*(1/3)}}}
                                                        ={{{80/3}}}
                                                        =26.67 km
Remaining distance to be covered  = 75-26.67 
                                  = 48.33 km
remaining time  = 1 hr  -20 min  = 40 min
                               ={{{40/60}}} hr
                               ={{{2/3}}} hr
              average speed to be maintained  =  {{{distance /time}}}
                                               ={{{26.67/(2/3)}}}
                                               ={{{26.67*(3/2)}}}
                                               =40 Kph

  Result is 40 kph