The associative property of addition is pretty much just any numbers in place of these: a + (b + c) = (a + b) + c. Therefore, your answer would be d) (6 + 1) + 9 = 6 + (1 + 9)
The four chairs bought in June cost $35 each, all together $140. Then they bought 2 more chairs on sale for 8% off of $35 each or 92% of $35 which is $32.2 each, all together $64.4. Both buy's together would then equal $204.4