Chapter 3 - Radian Measure and the Unit Circle - Section 3.3 The Unit Circle and Circular Functions - 3.3 Exercises - Page 119: 85c

$t=75^{\circ}$

Since May represents $x=4$, we substitute $x=4$ in the equation, $t=60−30\cos(\frac{\pi}{6}x)$ $t=60−30\cos(\frac{\pi}{6}\times4)$ $t=60−30\cos(\frac{2\pi}{3})$ $t=60−30(-\frac{1}{2})$ $t=60-15(-1)$ $t=60+15$ $t=75^{\circ}$