Effective interest rate ????

Jeeps co is concerned about its cash position and has taken to delaying payments to some suppliers in order to ease that problem. Each month the purchase ledger departments splits the total value of invoices for that month into 3 catergories
A, B, C depending on thier importance.

Category A invoices, amounting to $2000000 are urgent and paid after 30 days
Category B invoices, amounting to $3000000 are less urgent and paid after 60 days and
Category C invoices, amounting to $4000000 are least urgent and paid after 90 days

Several suppliers have reacted to this by offering Jeeps co a 2% cash discount if the accounts are settled
within 15 days. Jeeps co is currently considering whether or not to accept this. Another supplier who Jeeps co now
waits 90 days to pay, has been threatning legal action over the $300000 currently owed. Jeeps co feels that some sort of
compromise might be needed

Jeeps co cost of capital is 12% per annum
Assume that there are 30 days in a month and that purchases accrue evenly over the year

Question D
To avoid a court action, jeeps co is thinking of offering to repay the creditor it owes $300,000 in installments as follows (all figures in $000)
now- 75
1 month- 45
2 months- 45
3 months- 45
4 months- 45
5 months- 45

How much will jeeps co save, in present value terms, if the creditors accepts the instalment offer instead of jeeps having to pay in full immediately (to the nearest $00)?
I don’t get it why the solution has taken effective interest rate rather than simply dividing 12% by 12 i.e. 1%