Answer:
The formula for all b bins having at most 1 ball after k tossings that you wrote is:
[tex]\frac{(b)k}{b^{k} }\\[/tex]
So the probability that at least one collision occurs in k throwings (that at least one bin will contain at least two balls) is:
[tex]1-\frac{(b)_k}{b^k}[/tex]