Number 40 on the AP from Friday

40. So we set up the equation in class as

45=50e^(9k)

45/50=e^(9k)

.9=e^(9k)

ln.9 = ln (e^9k)

ln.9 = 9k

k=ln.9/9

Then you set up the second equation as

20 = 50e^(ln.9/9)t

20/50 = e^(ln.9/9)t

.4 = e^(ln.9/9)t

ln.4 = ln e^(ln.9/9)t

ln.4 = (ln.9/9)t

t = 78