Question 72870
This is hard to explain on here but I will try.

If you look at all the possible combinations:
1+1=2
1+2=3
.
.
.
1+6=7

then go to the 2's, you have already used 1+ 2 so now start with 
2+2=4
2+3=5
.
2+5=7
2+6=8

then go to the 3's, etc. 

When you finish with all the combos, up through 6+6. You will have a total of 21 combinations that could occur when rolling two fair dice.

If you look through them you will see that there are 3 combinations that will add up to 7.

So the probability that you will roll a sum of 7 is 3/21, which reduces to 1/7