Here is the solution:
Start with the fact that both trains have made the same distance= 's'.
In physics distance is a multiplication of speed of your train = 'v' and period of time in which it was moving= 'T' (this is only true when the speed doesn't change during movement, or we talk about avarage speed like in your task).
Then:
\(s= v_{D} * T_{D}\)
"D" symbol refers do diesel train
Freight train moves with 15[km/h] greater speed and it travels 1 [h] shorter then:
\(s = v_{F} * T_{F} = (v_{D}+15)*(T_{D}-1)\)
As the distance made by both train is the same you can compare the right parts of both equations:
\(v_{D} * T_{D}=v_{F} * T_{F} = (v_{D}+15)*(T_{D}-1)\)
Diesel train moved 6 hours and freight train 5 hours then:
\(v_{D} * 6=(v_{D}+15)*5\)
\(6*v_{D}=5*v_{D}+75\)
\(6*v_{D}-5*v_{D}=75\)
\(v_{D}=75\)
Here is your answer: The average diesel train speed is 75 km/h