Answer:
28 blue bikes
Explanation:
There follows a relatively simple method for solving this problem:
Let c be a counter. Then 4c + 5c = 63, and 9c = 63. Thus, c = 7.
Then the number of blue bikes is 4(7) = 28, and the number of black ones is 5(7) = 35. Note that 28 + 35 comes to 63.
There are 28 blue bikes.