sorry i still dun get it.

if PB for c+ is 2 mths, that means total 1.75+2=3.75, so c is 1.5mth means total 1.75+1.5=3.25

still dun see how the 4.5mth come about?? ior s B+ consider average performance? but i remember a cs friend told me to get B+ need to be top 20% performer?