Concentration of Reactant A After 20 Seconds

In a hypothetical reaction, the rate of conversion of reactants to products has been measured over time. Initial concentration of reactant A is 0.5 M, and the reaction follows first-order kinetics with respect to A. The rate constant (k) for this reaction has been determined to be 0.1 s^-1. If the reaction progresses for 20 seconds, what will be the concentration of reactant A at this time?