Borrowing costs

I am struggling to understand Example 1 covering borrowing costs in chapter 6. Seems I am lacking pure maths skills which drives me mad. Not quite sure why the example was not covered in the lectures either…What I don’t understand is the figures from the answer for “Invested” – 50,20,90,30,90 and “Spent” – 50,30,50,60,20,90. The first invested is fine, but we don’t have any 50 spent in the same period? Then in Feb there is no 20 invested? …or 50 spent in April…or 30 invested in May…or 90 invested in Aug?
Either I am lacking very basic maths or there is something wrong with the question.
Any advice will be much appreciated…Please, help…