Answer is D.
Tells us total delivery cost is $30.
So for first chair = 10 and remaining 20 chairs = 20 => Total chairs 1 + 20 = 21
So question can be answered by this statement alone.
Tells us average delivery cost of all chairs = 1.36
10 + (n-1)*1/n = 1.36 => n = 25
So question can be answered with this alone as well.
Do not worry if both options are giving different results, point to note here is that both can provide us the answer.
OA is B.
1 says only that 10+(n-1)>30 and that n>21. so we don't know whether n>24.