P(2,4,6) =1/2 , P(3,6)= 1/3 , P(1)= 1/6
So, P(2,4,6) +P(3,6) - P(6) + P(1) + P(5) =1
or, 1/2 + 1/3 - P(6) + 1/6 + P(5) =1
or, 1 - P(6) + P(5) =1
or, P(5) =P(6)
Now we know, P(3,6)=1/3
or, P(3) + P(6) = 1/3
or, P(3) + P(5) =1/3 [ since P(5)=P(6)]
or, P(5) =1/3 - P(3)
Since P(3)>=0, P(5)<=1/3