The AFL-CIO has undertaken a study of the yearly salaries (in thousands of dollars) of 30 administrative assistants. The organization wants to predict salaries from several other variables. The variables considered to be potential predictors of salary are:
XI = months of service
XI=years of education
X3 = score on standardized test
X4=words per minute (wpm) typing speed
X5 = ability to take dictation in words per minute
A multiple regression model with all five variables was run on a computer package, resulting in the following output:

Assume that the residual plots show no violations of the conditions for using a linear regression model.
a) From this model, what is the predicted Salary (in thousands of dollars) of a secretary with 10 years (120 months) of experience, 9th grade education (9 years of education), a 50 on the standardized test, 60 wpm typing speed, and the ability to take 30 wpm dictation?
b) What is the regression equation?
c) Test whether the coefficient for words per minute of typing speed (X4) is significantly different from zero at a = 0.05.
d) How might this model be improved?
e) A correlation of Age with Salary finds r = 0.682 and the scatter plot shows a moderately strong positive linear association. However, if X6 = Age is added to the multiple regression, the estimated coefficient of Age turns out to be bb = -0.154. Explain some possible causes for this apparent change of direction in the relationship between age and salary.