think of each character space of a box. the right most box is the '1' box, the second to right is the '2' box, the third the '4' box, the fourth the '8' box, and each box after that is just 2x the one to its right. now, if that box is filled (a 1 in that place) then you add the boxes numbers together. so since the 1, 2, 4, and 8 boxes are filled, you get 15.
but that was my logical explaination, there are TONS of websites that will do the work for you. try
www.google.com