P(getting a 1)=1/6 with 1 you can form 6,7 so p(sum>=6)=2/6
P(getting a 2)=1/6 with 2 you can form 6,7,8 so p(sum>=6)=3/6
P(getting a 3)=1/6 with 3 you can form 6,7,8,9 so p(sum>=6)=4/6
now every time we are going to form a number we require a particular number whose probability is 1/6
so total probability =1/6(2/6+3/6+4/6+1)=15/36=5/12