Question 884857
At 51kmh, the train would need to travel 1{{{1/3}}}hours to cover 68km.  [68-51= 17.  17 is {{{1/3}}}of 51.]
At 40kmh, the train would need to travel {{{1/2}}}hour to cover 20km [20/40]
Total travel time is 1{{{5/6}}}hours (or 1 hour 50 minutes).
In that time, total distance is 88km
88km / 1.8333 (converted the fraction to decimal for ease of use) = 48kmh average speed.