I need to calculate beta-galactosidase units for a lab and i am really having a hard time. please show me how to calculate.

Question

A B C
OD 600 .166 .109 .105

0d420
A1 .74
A2 1.40
B1 .16
B2 .33
C1 .10
C2 .11
average of 420 = 1.07-A, .25 = B, .15 = C, and .18 for Lb
Od550
A1 .89
A2 .11
B1 .07
B2 .21
C1 .10
C2 .11
Lb1 .11
Lb2 .18
Average of 550 = 0.50 =A, .14 = B, .11 = C, and .14 = Lb