the product of the 4 numbers is 0. that means that one or more of the numbers is 0. because they appear in defending order we know that the digit in the ones place is 0. the only common factor of 10 and 15 is 5 so we know that our number is 5 _ _ 0. because they add up to 9 the second 2 digits have to add up to 4. the only two numbers that add up to four and are not the same(they have to be in descending order so it can't be the same digit twice) are 1 and 3 so your license plate is 5310