the height, h cm, of a ball moving up and down in a harbour is given by h=30+20sintπ

where t is in seconds and the angle is in radians.

(i) find an expression for all the positive times at which the height of the ball is 20 m.

(ii) find the fourth time at which h=20. give your answer correct to three decimal places

so what i tried is 20=30+20sintπ

the answer at the back of the book says 1/6 +2(n) and 5/6 +2(n). i am not sure where this comes from. i do not know how to do part(ii)