Clarification Needed Regarding Method

hello ,
I was practicing FM working capital questions and I came across Receivables management question so I solved it but the method I used was as taught in the lecture videos and the answer I got was different than show in solution( The figure I got 504) vs the figure show in solution (580).

here’s the question :
Flit Co currently has total income of $30 million per year, of which 80% is from credit sales,
and a net profit margin of 10%. Due to fierce competition, Flit Co has lost market share and
is looking for ways to win back former customers and to keep the loyalty of existing
customers. The sales director has pointed out that a major competitor of Flit Co currently
offers an early settlement discount of 0.5% for settlement within 30 days, while Flit Co itself
does not offer an early settlement discount. He/she suggests that if Flit Co could match this
early settlement discount, annual income from credit sales would increase by 20%.
Credit customers of Flit Co take an average of 51 days to settle invoices. Approximately 0.5%
of the company’s credit sales have historically become bad debts each year and written off
as irrecoverable. The finance director has been advised that offering an early settlement
discount of 0.5% for payment within 30 days would increase administration costs by $35,000
per year, while 75% of credit customers would be likely to take the discount. The credit
controller believes that bad debts would fall to 0.375% of credit sales if the early settlement
discount were introduced.
Flit Co has an average short-term cost of finance of 4% per year. Assume that there are
360 days in each year.

Can tutor confirm if I made any mistakes and if not would I be losing marks for using this method since there is a difference in answer.