The table shows how the average age of first marriage of Japanese women varied in the last half of the 20th century.

$$\begin{array}{|c|c|c|c|} \hline t & A(t) & t & A(t) \\ \hline 1950 & 23.0 & 1975 & 24.7 \\ 1955 & 23.8 & 1980 & 25.2 \\ 1960 & 24.4 & 1985 & 25.5 \\ 1965 & 24.5 & 1990 & 25.9 \\ 1970 & 24.2 & 1995 & 26.3 \\ \hline \end{array}$$
Estimate the rate of change of marriage age for women in 1980 by averaging the secant line slopes for the years 1975 to 1980, and 1980 to 1985.

Answer: