math
posted by Nora .
The number of cases for a disease has reduced by half every 10 years:
1970  21,168
1980  10,584
1990  5,292
Assume N is the number of reported cases t years after 1970. Which is a good model for N(t)?
a) 21,168 (2)^t
b) 21,168 (1/2)^t
c) 21,168 (1/2)^t/10
d) 21,168 (2)^t/10

You need t/10, since the number is halved every 10 years.
Since it's halved, that's (1/2)^(t/10)
So, C