5 times as many. Since dog B has 2 and dog A has 10, A = 2 * X, since you’re solving for the multiplication comparison. 10/2 = 5, so you’ve got 5 times as many.
5 + 8s = 15 + 3s
8s - 3s = 15 - 5
5s = 10
s = 10/5
s = 2 <=== they will charge the same at 2 sq ft