Language according to "carefree, help others, joy begets sorrow, parting is reasonable, the end of the road, know horsepower, and strive for gold day by day" to answer.

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.