Answer:
 5.5 minutes
Step-by-step explanation:
Fill in the given information and solve for t.
 75 = -15cos(2πt/15) +65
 -10/15 = cos(2πt/15)
 t = 15/(2π)arccos(-2/3) ≈ 5.492
The volume reaches 75 dB about 5.5 minutes after the truck arrives.