Find the half life of the number of polio cases

Question

1988	350
1992	138
1996	33
2000	4
2005	3.2
2007	1.3

YEAR Number of Polio cases (thousands)

Find the approximate half life of the number of polio cases

Answer 1

1.3 = 350e^19r

1.3/350 = e^19r

take natural logs of both sides, then divide by 19

r = ln(1.3/350)/19

r = about -.2945= annual percentage decrease = 29.45%

1/2 = e^-.2945t

t = ln.5/-.2945

t = 2.778 years = half life