Question 681441
Generally it is an exponential function.
It may be D= 5e^(-0.37h)
I assume the above
here D = 2 
2= 5e^(-0.37h)
/5
0.4=e^(-0.37h) 

taking the natural log 
 ln(0.4)=-0.37h
 (ln(0.4))/(-0.37)=h 

h is the time in hours