Your question was –
If the cost of one dozen of apples is $96 and the cost of 1 dozen of bananas is $36, then find the average cost of 18 apples and 18 bananas?

A few days ago I take part an exam where I have faced this GMAT questions and my answers option were –

a. $3
b. $8
c. $12
d. $5.5
e. None of the above

I have answered as my Correct answer is =  d

Because of this following Explanation:

According to the question,
The cost of one dozen of apples = $96…..i
The cost of 1 dozen of bananas = $36….ii
We know hat, one dozen = 12 units
∴ From statement (i) we can say that, 12 apples = $96
∴ Cost of 1 apple = 96/12 = $8…iii
∴ Cost of 18 apples = 18*8 = $144…..iv
Similarly, from statement (ii) we can say that, 12 bananas = $36
∴ Cost of 1 banana = 36/12 = $3…..v
∴ Cost of 18 bananas = 18*3 = 54……vi
Hence, the total cost of 18 apples and 18 bananas = $144 + $54 = $198
But, we have to find the average cost of 18 apples and 18 bananas = 198/36 = 5.5

Hence, the correct answer is option d.

Hope you have got your answer.
Thanks.