You've been given a data point for the decay equation, ; namely, (t, A) = (1, 0.73), where t is in hours. And they've given you the value of P; namely, 400.

So plug the values they've given you into the equation they've given you, and solve for the decay rate r. Then plug in "200" for A (that is, half of P), and solve for the time t.

If you get stuck, please reply showing your work and reasoning so far. Thank you!