(Material from Appendix II in the Text)

For Mary: N_m = N₀e^+1.0t

For Joan: N_j = N₀e^-2.0t

Find t for which N_m = 4N_j

or, N₀e^+1.0t = 4 N₀e^+2.0t

e^+1.0t/e^-2.0t = 4
take natural logs
3.0t = ln 4

t = (ln4)/3.0 days

Answer is E