Each subject is worth x number of units (mostly 2).
Each unit is worth 50 marks.
Therefore for each 2 unit course you have 2 equal marks /50.
Thus is it the 10 best unit marks out of 50 that are used e.g.
2 unit English - 80% = 2 x 40
2 unit Maths - 90% = 2 x 45
2 unit Modern - 96% =2 x 48
2 unit Geography - 94% = 2 x 47
2 unit Society - 88% = 2 x44
1 unit SOR - 46
Therefore - 2 units of English (2 x 40), 2 units of Modern (2 x 48), 2 units of Geography (2 x 47) , 1 unit SOR (1 x 46), 2 units of Maths (2 x 45) and 1 unit of Society (1 x 44) would count.