Samuel paid $9.10, including tax, for a dog collar. Before tax, the original price of the collar was $8.75. What was the sales tax rate?
The sales tax amount is $9.10 - $8.75 = $<<9.10-8.75=0.35>>0.35.
The sales tax rate is ($0.35 / $8.75) * 100% = 4%. Answer: \boxed{4}.