Effective Cost percentage calculation, ACCA FM (F9)

I’m in utter confusion regarding the topic. I have come across 2 different ways/ formulae to calculate the effecetive cost percentage in the “Managing recievable” chapter. The first one I came across was in the BPP notes where; I’ll use the following example for a clear understand;

A company normally recieves payment after 3 month. It has proposed to offer 4% discount for customers that pay within 1 month. Calculate the effective cost % p.a.

Using the BPP notes they simply did the following:
4% / 96% * 12/2 = 0.25 (25%)

I came across a different method in the OpenTuition notes:

1+R = (1+(4/96)^12/2)
Which gives us = ((1.041667)^12/2 – 1 = 0.2775 (27.75%)

I’m not sure which one to use.