Hi dear,

Aggregate 50 percent means your total number in exam divide by total number of subject. If it is 50 or more then your aggregate is 50 percent. If there is 50 percent aggregate in PCM or PCB means total number in PCB or PCM divide by 3. Let your physics marks is 60 in chemistry 55 and in math 52 then aggregate percent in PCM is 60+55+52/3=55.6

Hope it helps.