If you didnt know the formula for evenly spaced sets, you can pick numbers and solve this question very easily.
Pick first number of set T=2. Since number of elements = 5, the mean will be the middle term ie the 3rd term, which will be 6.
From the question, you can infer that first number of the set T, will be 2+7= 9. Since this set consists of even number of terms,ie 10, the mean will be the average of the middle 2 terms, ie 5th and 6th term, which are 17 and 19 respectively and their average will be 18= mean of the set.
Therefore, the difference between the mean of two sets= 18-6= 12.
My GMAT debrief: from-620-to-710-my-gmat-journey-114437.html