On a 120 km track, a train travels the first 30 km with a uniform speed of 30 km/h. How fast must the train travel the next 90 km so as to average 60 km/h for the entire trip?

Step 1, Given data
Total distance $=120 \mathrm{km}$
$30 \mathrm{km}$with a speed $30 \mathrm{km}/\mathrm{h}$
Average speed $=60 \mathrm{km}/\mathrm{h}$
Step 2, Finding the speed
average speed during the entire voyage $=60 \mathrm{km}/\mathrm{h}$
Consequently, the whole travel time will be, $T=\frac{120 \mathrm{km}}{60{\mathrm{kmh}}^{-1}}=2 \mathrm{h}$
Time spent during the initial portion of the voyage, $\mathrm{t}=\frac{60 \mathrm{km}}{60{\mathrm{kmh}}^{-1}}=1 \mathrm{h}$
Duration of the second leg of the voyage $=T-t=2h-1h=1h$
Distance travelled in the second leg of the trip is equal to the sum of the distances travelled in the first and second legs $=120 \mathrm{km}-30 \mathrm{km}=90 \mathrm{km}$
Hence, the second half's average speed$=\frac{90 \mathrm{km}}{1 \mathrm{h}}=90{\mathrm{kmh}}^{-1}$

Hence, the correct answer is option B.