Borrowing Costs – a simple doubt

Hi all,
Hope you all are ready for the exam.
I have a doubt regarding Boorowing costs. Pls check this site

https://www.nelsoncpa.com.hk/slidepdf/ACCA-20070703.pdf
On that on pg 63.. Rechno co
They have taken the borrowing cost only on the amount used.. Though the whole amount was borrowed. So how can they consider the investment income obtained from borrowing that money but not take the borrowing cost for the whole time

This question is an example in BPP and the same method is used.

Thanks and all the best for the exams!