for example your question is, "the sum of three consecutive numbers is equal to 30, what is the largest number"
we have three unknown numbers, but we know something about their relationship to each other, they are consecutive numbers, so one numbers come right after the last one, like 1, 2, and 3, or 1, 1+1, and 1+2. We also know that the three numbers added together equal 30. So now that we know how our numbers interact with each other, we have to assign a variable. If we assign the lowest number as the variable "x" and we know that the other two numbers follow one right after the other, we can deduce that the equation should look like this
so next we combine like terms, using the communitive property of addition, we can rewrite our equation to look like this
then we subtract 3 from both sides
then we divide both sides by 3
so now we know what the smallest number of the three is equal 9, but if you look at the question again, it's not asking what the smallest number is, but the largest number, but because we know that the largest number is x+2, we know that the largest number is equal to 11 and we're done