If the average of a set of 6 numbers is 9, the average of first four is 10 and that of last four is 8. Then find the average of the third and fourth number.

Let the six numbers be a,b,c,d,e,f. Total sum = 6×9 = 54. First four sum = 4×10 = 40, so a+b+c+d = 40. Last four sum = 4×8 = 32, so c+d+e+f = 32. Adding these: 2(c+d) + (a+b+e+f) = 72. Since total is 54, subtracting gives 2(c+d) = 72-54 = 18, so c+d = 9. Their average = 9/2 = 4.5. Wait - the question asks for average of third and fourth number, which are c and d. If c+d = 9, then average = 9/2 = 4.5. But option C is 9, which is the SUM, not the average. Let me verify: If c+d = 9, average is 4.5. None of the options match this. However, if we misread and the question wants the SUM of third and fourth (not average), then 9 is correct. Given option C is marked correct, there may be a wording issue or I'm misinterpreting.