Answer:
60 km/h
Explanation:
You need to know how long it actually took for them to reach Y. So the length of B's route (60 km) divided by the average speed (40 km/h) equals 1.5 hours. Since you know how long it took to get to Y, you divide the length of A's route (90 km) by the time it took to get to Y along B's route 1.5 hours). This means 90/1.5 which gives you 60. This is the average speed for A's route.
You can check it by dividing the speed of a route by the length of that route which should both give you two thirds: 40/60 = 0.6666666667 don't exactly know how many sixes).
And 60/90 = 0.6666666667 (again, check the amount of sixes, but it's the same as two thirds= 2/3).