6.

(a) dM/dt = (0.50 + 0.25t)M
(b) M = 100,000e^{0.50t+0.125t^2}.
(c) About 27,727,000 cruzieros.

7.

(a) r is about 112.5%
(b) Graph! Maybe later if wanted.

8.

a = ge^-kt/m

13.

(a) dy/dt = -k(y-a), where k > 0 and a are constants.
(b) y = (1-a)e^-kt+a
(c) a represents the fraction of material is remembered in the long run.

16.

t = -ln(0.98)/(3 * 10^-5) is about 673 minutes.