Answer:
$1,539.70
Explanation:
Given the data :
National average Teachers' salary (NATS)
YEAR:_1960 _ 1970 _ 1980 _ 1990 _ 2000 2010
NATS:4995_8626_15970_31367_41807_55202
AVEAGE SALARY INCREASE PER YEAR BETWEEN 1980 and 1990
Salary in 1980 = $15970
Salary in 1990 = $31367
Average salary increase per year:
Difference in salary / difference in years
$(31367 - 15970) / (1990 - 1980)
= $15397 / 10
= $1539.7