c.) 5 = 2 sin pi/6 t + 4
2 sin pi/6 t = 5 - 4 = 1
sin pi/6 t = 1/2
pi/6 t = arcsin (1/2) = 2n pi + pi/6, (2n + 1)pi - pi/6 = pi/6(12n + 1), pi/6(12n + 5)
t = 12n + 1, 12n + 5, n = 0, 1, 2, . . .
t = 1, 5
Therefore, the depth is 5 at 1 a.m. and at 5 a.m.