First, list all the sums two dice make when they are rolled, e.g. 1+1=2, 1+2=3, ... , 6+6=12.
The following is the set of sums listed in ascending order: {2,3,4,5,6,7,8,9,10,11,12}.
Then, list the numbers: how many combinations each sum has, e.g. 1+4=5, 2+3=5, 3+2=5, 4+1=5 so the sum "5" has 4 combinations. The following is the set of numbers of combinations listed corresponding separately to each sum in the set above: {1,2,3,4,5,6,5,4,3,2,1}.
So, total number of combinations is 36, and the sum "7" has 6 combimations. Thus, the chance the sum of two dice makes "7" is 6 times when dice are rolled 36 times. The probability is 6/36 in 36 trials: approx.16.67%. Therefore, the probability of "7" in 35 trials: (6/36)(35/36)=35/216
The answer: The probability is 35/216 in 35 trials (=approx.16.20%)