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$T_{old}$ = 25 hours, and$T_{new}$ = 1500 / 150 + (2500 - 1500)/100 = 20 hours. And the speedup for the trip is 25/20 = 1.25. - if we want to get an overall speedup of 1.67*, we need spend 15 hours in this trip. Because of speed limits, we had spent 10 hours in 1000km, thus we use spend 5 hours in unlimited road, which constitutes 1,500km, and it's 1500km/5h = 300km/h.