Since I don't know what this number is, I'm going to use the variable x to substitute for the value of the unknown number.
So it would just be x + 15 because addition is the operation of adding and the key words are usually more. Since one of the numbers is unknown, we can't get an answer like "100" because x is undefined.
Say x=100, then the number would be written as 115 in it's simplest form but you could also write it as 100+15. So I'm just using substitution.
The probability of rolling less than 7 is equal to the probability of rolling 1 or 2 or 3 or 4 or 5 or 6. That is 1/6 + 1/6 + 1/6 + 1/6 + 1/6 + 1/6 = 6/6. So any number from the dice can come up when you roll it because a perfect dice has 6 sides.
