How many subsets of (1; 2; 3; ... 14) have 15 as the sum of their largest and smallest elements in the subset?

(In reply to solution (spoiler) by Charlie)

Beautiful!

Still, staying decimal , one can  see it as a  gometric sequence <br>

with a(o)=4 and q=4 so the sum is (surprise,surprise ):<br>

 

(4^7 - 1) / (4-1)= 5461

isomorphic solution

Edited on May 16, 2010, 2:43 pm

Posted by Ady TZIDON on 2010-05-16 13:58:09