Remember I said that when you have an average problem and they are talking about a change in average, take it out to the totals. That's where these problems are vulnerable. It says the average of A, B, S, and T=6; therefore (A+B+S+T)/4=6, so A+B+S+T=24. The average of S+T=3, so (S+T)/2=3; therefore, S+T has to equal 6. Now, if we subtract S+T from A+B+S+T, we're left with A+B. The value of A+B would then be 18, and therefore, (A+B)/2=9. So the average of A+B is D, 9.