This is an exterior angle property, one of the properties in a triangle. The exterior angle theorem states that if a triangle's side gets an extension, then the resultant exterior angle would be equal to the sum of the two opposite interior angles of the triangle.
End of First Year (750/100)x12(3/5)=54 $750+54=804 End of Second Year (804/100)x12(3/5) = 58 $804+58=862 End Of Third Year (862/100)x12(3/5) = 62 $862+62=924