Probability=P
P=what you want
--------------------------
total amount
1,2,3,4,5,6,7
1+2= no
1+3=no
1+4=no
1+5=no
1+6+yes
and then you go on like that from there
(sorry i didn't realize how long this would take but you can make a number grid and just count out the sums and go from there)
Hope this helps!!!