-- If the number is divisible by 15, then it's divisible by 5 .
----- So it must end in 5 .
----- Since it's palindromic, it must also begin with 5 .
Once I reached that point, my brain was worn out from the strain,
so I did the rest of it the cheap and easy way.
I took my calculator, started at 6,000 (a multiple of 15), and kept
subtracting 15 at a time until I saw a palindromic number.
The first one I saw is 5775 . So that's the largest 4-digit
palindromic multiple of 15.
The sum of its digits is (5 + 7 + 7 + 5) = (12 + 12) = 24 .