first order reaction

rate constant at 427k: 5.11x10^{-5}

initial concentration is 0.0340M

I know half life: 0.693/5.11x10^{-5} = 1,3561sec --> 3.7hour

Is there a method for solving these above questions or is it just about estimation of what you know?

question 1: what is the molarity after 2.00hour?

I got about 0.0183 molar

question 2: how many minutes does it take for concentration to drop to 0.0300M?

I got about 1 minute

question 3: how many minutes does it take for 20% to react?

I got about 90.4 minutes