Managing Account Receivable Question

Hi ,

I have one question not understand,on BPP F9 2016 page 101.

Question :
winterson tools has an average level of accounts receivable of $2m at anytime representing 60 days outstanding ( their term are 30days) . The firm borrows money at 10% a year. the managing director is proud of the credit control .: ” I only had to write off $10,000 in a bad debts last year:, they said proudly . Are the right to be proud ?

Answer from BPP :
The managing director may be proud of the low level of bad debts but customer are taking an extra month to pay and this has a a cost . At any one time, there is 1m more money outstanding than there should be ( 30/60days x $2m). and this cost &100,000 in interest charge ( 10% x &1m)

1) I’m not understand the logic behind how does the they count the $1m more money outstanding , why divide 30/60days x 2m ?

2) the interest rate is per annum, the outstanding were just one more month, why shouldn’t it be ( 10%/12month x $1m) ?

Thanks !