This math problem is as follows:
From the meaning of the question: A runs for 4 hours, and B needs to run for 5 hours. So the ratio of a and b to speed is 5: 4.
Let the speed of A be 5X and the speed of B be 4X, then the whole journey is (5x+4x) × 4 = 36x.
Then the overall running time of A is: 36x ÷ 5x = 7.2 hours.
In other words, after running the race in 7.2 hours, A ran 90 kilometers in 1.8 hours.
So the speed of A is 90 ÷ 1.8 = 50 kilometers per hour.