The example you've cited, does not provide a complete picture of the question you have. (r-8)*2
can be sufficient or insufficient based on whether you know the value of 'r'. If you can somehow determine the value for variable 'r', using the question stem and the given statement alone, then your statement should be sufficient.
Let me put it this way. Assuming that the question you've posted is something like this:8 years ago, Person A's age was twice that of Robert's. What is Person A's current age?
Statement 1: Robert's age will be 15 in 5 years time.
Given this information, we know that Robert's age (let's say, r) is r = 10 years and using the question stem, we know that Person A's age, 8 years ago, was given by the equation: (r-8)*2
Without calculating, we can definitely say that Statement 1 is sufficient as we know the value of variable 'r' and hence, can determine Person A's current age. Hence sufficient.
Whether or not a given statement is sufficient will also determine how you frame your equations. In certain cases, we tend to use more number of variables than is actually required. Due to this, we end up getting equations with at least 1 unknown variable value, which cannot be solved/reduced further to determine the correct solution.
My advice would be to read the question stem very carefully and then frame the necessary equations using the minimum number of required variables. Then look at each of the given statements separately to determine whether they are sufficient. In most cases, you won't be required to solve the equations - you could determine whether the statement is sufficient, just by looking at the information presented in the question stem and the statement.
Hope this helps!
MBA Candidate 2015 | Georgetown University
McDonough School of Business