Translating "The tax is some percent of the phone's price" into an equation, we get:
42.42 = p · 499
If we divide both sides by 499, we get
0.085 ≈ p
We can check by finding 8.5% of 499:
0.085 · 499 = 42.415, which rounds to 42.42
So the tax rate was 8.5%.