– Calculate his interest after 65 days: $6000 (principal) x .5 (interest rate) x 45/360 (days) = $ 37.5

– Subtract this interest from the payment made on day 65: $2100 – $37.5 = $ 2062.5

– Since we have subtracted the interest paid on this partial payment, the remainder of the payment goes toward paying off his principal: $6000 – $2062.5 = $ 3937.5 (adjusted principal)

– Calculate interest on day 75 (45+50), which is 30 days later: $ 3937.5 (new principal) x .05 (interest rate) x 30/360 = $13.125

– Subtract this interest from the payment made on day 89: $1800 – $13.125 = $1786.88

– Again, the remainder of the payment goes toward paying off his adjusted principal: $ 3937.5 – $1786.88 = $ 2150.625 (adjusted principal)

– He still has 45 days left on this 120-day note (120-75), so now we calculate the interest owed on the last 31 days: $ 2150.625 x .05 x 45/360

= $13.44 interest owed

– His final balance due will be the adjusted principal plus the final interest owed: $2150.625 + $13.44 =   $ 2164.066

– Total interest due will be the sum of the 3 interest payments: $ 37.5+13.125+13.44 = $64.06