The average rate of change is defined as:
[tex]\frac{f(b)-f(a)}{b-a}[/tex]using the information given
a=1999
b=2004
f(a)=1.3
f(b)=1.5
then,
[tex]\begin{gathered} \frac{1.5-1.3}{2004-1999} \\ \frac{0.2}{5} \\ 0.04 \end{gathered}[/tex]The average rate of change between 1999 and 2004 was 0.04 billion.