Your question was –
If the average cost of 5 commodities is 20 units and the average cost of 3 of those commodities is 15 units, then find the average cost of the remaining two commodities?

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

a. 20.5
b. 21.5
c. 22.5
d. 26.5
e. 27.5

I have answered as my Correct answer is = E

Because of this following Explanation:

According to  the question,
The average cost of 5 commodities = 20 units……i
The average cost of 3 of those commodities = 15 units……ii
We have to find the average cost of the remaining 2 of those commodities.
Let the commodities be a, b, c, d, e
Hence, from i we can say that, a+b+c+d+e/5 = 20
∴ a+b+c+d+e = 20*5 = 100 ……iii
Also, from ii we can say that, a+b+c/3 = 15
∴ a+b+c = 15*3= 45 …..iv
Now, substituting equation iv in equation iii, we get,
45+d+e = 100
∴ d+e = 100 – 45
∴ d+e = 55
But, we have to find their average. Hence, dividing both sides by 2 we get,
d+e/2 =55/2 = 27.5
Hence, the correct answer is option e.

Hope you have got your answer.
Thanks.