It's very important to write the equation properly. The way your equation is written originally there is no period at all. Your left side function of the variable "t". Since t doesn't show in the right side, the right side is a independent of t. Hence it's a constant and therefore the period is infinity.
Jim Clay already fixed that in his response. The other potential pitfall are units. If you write the equation as sin(2*pi*(1/20)*t), then t is a unit-less quantity since you cannot take the sin() of a quantity that has units. So the answer would be that the period is 20 (and not 20s). If you write the equation as sin(2*pi*(1/20s)*t), then the t is a time and the units of t cancel with the units in the term (1/20s). In this case the period would be 20s.