Assume Subject X has 2 papers, P1 and P2. P1 has 40 marks and p2 has 60 marks. If in June 2018 CAIE sets the minimum number of marks required for an A in p1 as 32/40 and for a B as 29/40. Likewise it sets A at 52/60 in P2 and B as 48/60 in P2. So, you need a minimum of this to achieve the required grade in each paper. Now, CAIE will total your number of marks. One thing is for sure the minimum required to get A* will be set as many marks above the sum of the Grade A minimum as the grade B is below it. In this example:

for B: 29 + 48 = 77/100

For A: 32 + 52 = 84/100.

Now CIE will subtract the minimum for a B from A.

Hence, 84-77 = 7 marks.

These will be added to the sum of A grade marks i.e. 84 + 7 = 91 marks. Therefore, 91/100 will be the minimum for an A*.

So, you can see it is not necessary to get a minimum in each paper. If you got say 31/40 in P1 it'll be given grade B according to our assumptions above. And if you got 60/60 in P2 you'll get an A in p2. But the total will be 31 + 60 = 91 marks. Hence, you've achieved the minimum for A*.

The minimum for A* that is 91/100 in our example will be mapped onto a percentage uniform mark of 90%.

The minimum for A in our example 84 will be mapped onto a percentage uniform mark of 80%.