Setoff Rules

I AM TRYING TO UNDERSTAND THE SETOFF RULES. SO SOMEONE PLEASE CLARIFY THIS EXAMPLE

If  student A gets 240 in Group 1 say equal 60 in all 4 subjects

and 175 in group 2,

so total 425

but say he gets 35 in costing i.e he fails in costing in group 2

and in no other subject in group 2 does he get more than 60 i.e. no exemption

So he has to give whole group 2 again  in next attempt. 

Now he does not get the 40 marks advantage as setoff in his next attempt

If student B gets 210 in Group 1 say equal 60 in 3 subjects and 30 in last subject i.e. 

and 175 in group 2, (exactly same as student A)

so total 385

So he has to give whole group 2 again  in next attempt 

and he has to get only 40 in the last subject in group 1 which he failed i.e. the last one coz he can get advantage of the three 60s as setoff

BUT say IF HE GETS 60 IN THIS LAST SUBJECT OF GROUP 1, HE CAN GET SETOFF OF THE EXTRA MARKS HE GETS IN THAT SUBJECT AGAINST THE OTHER GROUP?

subjects in which he got 64.