24054

this is a solution to challange four.First the key number is 15. first put number 1 in the in the right middle square because that is the only place it can go as there is only one way of making 15 with 1 which is 1+8+6=15. Then find out what adds to 1 to make 15 which is 6 and 8. then find options to make 15 using 8 or 6. if you choose 8 you will get 3+4+8=15 if you choose 6 it will be 7+2+6=15 then the left side will be 7+5+2=15. And there is the answer to challange 4 ... Good luck and i hope you found it usefull.