3. A school secretariat wants to understand the influencing factors of the job satisfaction score (Y) of its contracted employees. Consider four explanatory variables (X1, X2, X3, X4):
X1: Years of service (years).
X2: Salary level (thousand yuan).
X3: Whether you have a bachelor degree, X3=1 means you have a bachelor degree; X3=0 means no bachelor degree. (BA)
X4: Master's degree or above, X4=1 means master's degree or above; X4=0 means no master's degree or above. (MA)
Now, the data of 30 contracted employees are obtained, and the dependent variable Y is regression analysis for four independent variables (X1, X2, X3, X4), and the estimated regression model in the following table is obtained.

Note: SST = 989.7; s = 4.8. SST represents total sum of square, and s represents the estimate deviation of the standard error of regression. Please answer the following five questions. (25 %)