Thank you for the opportunity to help you with your question!

The ordinary interest is calculated as; [p x r x d/360 ]

where d is the no. of days

The ordinary interest till 40th day on $8500 is = $137

so the total sum will be 8500+ 137 = $8637

Raul paid $4250 on the 40th day , so the remaining amount is 8637- 4250 = $4387

now the rest of the interest shall be calculated on this amount: that is $ 35.33 for 20 days

so the remaining amount to be paid on 6oth day is $4422.33

total amount paid = 4422.33+ 4250 = $ 8672.33

if he would have paid the entire amount on 60th day then he would have pay the interest of : 205.41

sothe amount would be 8500+205.41 = 8705.41

thus he saved 8705.41 - 8672.33 = $33.08 ($33)

Please let me know if you need any clarification for the doubts. I'm always happy to answer your questions. Looking forward to help you again. thank you. :)