right, you just have to read the question properly and organize it:
Car: does 75km in t minutes at a speed s
Train: does the same 75km in t-12,5 minutes (as it stopped for 12,5mins) at a speed 1,5s (as it can go 50% faster than the car)
as speed is distance/time, if you match the speeds:
1.5*75/t = 75/(t-12.5) <=> t = 37.5 mins which gives s = 75/37.5 = 2km/min = 120 km/h.
hope it helps, although you wont really find questions of this type (so elaborate) during the tests. You should practice head ons etc. which are generaly easier