What is the time when a person is 15 m above the ground?
h= 20sin 2pi/40 (t-10) + 21
well, that would be when
20sin 2pi/40 (t-10) + 21 = 15
20sin 2pi/40 (t-10) = -6
sin 2pi/40 (t-10) = -.30
sin .304 = .3, so
2pi/40 (t-10) = pi+.304 or 2pi-.304
t = 31.935 or 48.065
or other values periods away.