The following output was obtained.

Analysis of variance
SOURCE DF SS MS
Regression 5 100 20
Residual Error 20 40 2
Total 25 140

Predictor Coefficient SE Coefficient t
Constant 3.00 1.50 2.00
X1 4.00 3.00 1.33
X2 3.00 0.20 15.00
X3 0.20 0.05 4.00
X4 –2.50 1.00 –2.50
X5 3.00 4.00 0.75

a. What is the sample size?