Hey there!
If we start out with the same amount of boys and girls at the beginning, then we must add the same number to both groups to maintain that same amount. In this case, since there's already 4 in both groups, you would need to split 2 in half and add the remainder to both groups.
2 ÷ 2 = 1
boys: 4 + 1 = 5
girls: 4 + 1 = 5
So, 5 girls and 5 boys are running.
Hope this helped you out! :-)