To get average speed you take:
Total distance / total time = average speed
As we can see, the total distance that the motorist travelled was:
[tex]60km+90km=150\operatorname{km}[/tex]And, the total time taken is:
[tex]1h+2h=3h[/tex]Then, his average speed is:
[tex]\frac{150\operatorname{km}}{3h}=\frac{50\operatorname{km}}{h}[/tex]