I assume it's the same as the Egyptian Tansee2!
A* = 100
A = 95
B = 85
C = 67.5
D = 60
E = 52.5
Lets say you did 8 O-levels and scored 5A* and 3A and 2 Alevels where you scored 2 A's, your calculation will go like this:
(5 x 100) + ( 3 x 95) + (2 x 95) and then you'd divide the total by the number of subjects [10] and then you'll get your score!