The motorist travel 2 hours at a speed of 120km/h -> In 2 hours, he/she travel : 120 x 2 = 240km
The next 2.5 hours, the motorist travel at 102km/h -> In 2.5 hours, he/she travel : 102 x 2.5 = 255km
So, he/she travel 255 + 240 = 495km in 4.5 hours
-> His/her average speed is : 495 : 4.5 = 110km/h.