Trigonometry|Precalculus

Question

An ice cream truck that plays loud music is circling Bulan's neighborhood. C(t) models the volume of the music (in dB) that Bulan hears, t minutes after the truck arrives in her neighborhood. Here, t is entered in radians.

C(t) = −15cos (2π​/15 t) + 65

How many minutes after the truck arrives does the volume first reach 

75 dB?

Round your final answer to the nearest tenth of a minute.

Answer 1

75 = -15 cos (2πt / 15) + 65

10 = -15 cos (2πt / 15)

-2/3 = cos (2πt / 15)

2.30 = 2πt / 15

34.51 = 2πt

17.25 = πt

5.49 = t