Answer :
5100 = 4000e(6r) . We want to solve for r; therefore, In(5100/4000) = 6r In(e) = 6r; therefore, r = .243/6 = .0405. Now plug this value back into the equation for the time from 6am to midnight or 18 hours. So we have 4000e^(.0405 times 18 hours) = 8,291.